Backslash key no backslash key

on a standard keyboard should be key under backspace key

whats the operating system

Check the Regional Options of your pc, under the input tab

ASCII - Hold the Alt key and type 92 is \ backslash Alt - 47 is / forward slash.
You may have reassigned the keys. Go to Control Panel and reset in Region and Language > Keyboard.

the keyboards are set up on international standards and it is not usual to be able to configure a separate key for some other symbol

what is the keyboard type you are using?

How do I access the backslash key on an american keyboard

1) With your NUMLOCK on, hold the ALT key down and type 092 on your numeric keypad.  When you have NUMLOCK on, ALT and a 3-figure digit gives you special characters.  092 is the backslash on UK and Spanish keyboards, so hopefully it'll work for you.  If you get something different, you'll have to try different numbers until you find it

or 

2) In Start --> Programs --> Accessories there's a program called Character Map.  This gives you a list of available characters

or

3) copy and paste this one:  \

Hello,
For backslash [2nd][2]
Negative [(-)]

Hope it helps