Surd answers on the fx-991ES plus

Change the output display to MathIO. You may be able to display square roots only: so no cube roots, nor fourth...

Press the Shift/Mode key and set the Input/Output to MathIO.
Even that will not guarantee that the display will be in root form. Only square roots are possible.

PDF]fx-570MS/991MS Additional Functions - Support - Casio

support.casio.com/pdf/004/fx570MS_991MS_E.pdf

Casiodisplay format (Disp) settings while the calculator is in the BASE ... Be sure to checkthe current calculation mode (SD, REG,. COMP ... Use [ and ] to display the expression 4 + 4. Press A ... want to perform scientific function calculations. COMP .

The Casio fx-991es, fx-570es and - Numericana

www.numericana.com/answer/casio.htm

The Casio fx-991ex succeeds the fx-991es (functionally identical to the ... Both are nonprogrammable nongraphing scientific calculators, with natural ... Side-by-side comparison of the fx-991EX and fx-991ES PLUS. ... Displays QR codes for smartphone use .... Work out a formula for many values of the input variables.

CASIO fx-991MS Calculator Tutorial: Lesson 5- Dealing with ...

Looks like <Shift><Mode> <down arrow> <1>

Give it a shot. I have attached the manual.

Good luck.

Paul
http://support.casio.com/pdf/004/fx-115ES_991ES_Eng.pdf

Your answer is 6.7*10^18. That requires 19 places on the display: There are not enough places to write that. Besides, calculators are deigned to use Scientific Notation whenever the result to be displayed is too large or too small.
If you enjoy writing zeros, convert by hand the scientific notation into what you call long number.

This is a good scientific calculator that can do a lot, but things in the realm of a CAS (Computer Algebra System) are beyond its capabilities.

Select the Math input/output mode by pressing SHIFT [SETUP] 1.

[SETUP] is the shifted function of the MODE key, located just right of the round cursor pad.

The capability of these calculators to handle irrational numbers in radical form is limited to square roots. Roots of index other than 2 are always displayed in their decimal representation.
If the calculator IO mode is set to MathIO, square roots will be displayed with the radical symbol. More complicated expressions with square roots might not be preserved and will be given as decimal numbers.

To convert a single answer from symbolic to decimal, press the S--D key (just above the DEL key). To have all answers come out in decimal format, press SHIFT MODE 2 to switch to the LineIO mode.

The calculator can't identify that this is a surd unless you input it as a surd, ie as 3/(4*sqrt(5))= when it will then show the equivalent form 3*sqrt(5)/20 (obtained by multiplying top and bottom by sqrt(5)) and you can swap between the two forms using the S<>D button. Even adding the next 4 decimal places (hidden) won't allow the calculator to identify the underlying surd as this is simply too complcated a calculation.