claire,

you can find LCD routines in the code library.

You can calculate as you would with paper and pencil:
1)Do you have the first operand, the operation and the result.
2)Get the user input as the second operand and do the operation with the first operand. If the result of that operation matches with the result in the display then the user guess is correct, if don't it's the wrong answer.

HTH

orlando