My canon P170-DH won't add or subtract. it just continues putting numbers on the screen

A canon P170-DH appears to be a printing calculator, not a computer motherboard. From the description you've given, it would appear a key is stuck in the down position or possibly a sticky liquid such as "pop" has infiltrated the keypad and caused a contact to be held "stuck" in the closed position as if one were continually pressing a key. Some moderate level of skill would be necessary to take the unit apart so that it can be inspected further. First the unit must be unplugged for safety. I suggest taking photos with a digital camera at different stages of the taking apart so as to show original key placement should they all just come out and get mixed up as the unit is taken apart. If the key contacts are comprised of a membrane type keypad, it may not be possible to repair the existing keypad and getting such parts as a keypad for a sub $100.00 calculator could become an issue. With shop repair rates in the range of 60$/hr, the demand for parts may preclude any availability as the issue of professional economical repair becomes unlikely.