For equations:
* Find - "Find the value of x."
* Determine - "Determine the solution to the equation."
* Calculate - "Calculate the answer."
* Evaluate - "Evaluate the expression."
* Simplify - "Simplify the equation."
* Isolate - "Isolate the variable."
For problems:
* Answer - "Answer the problem."
* Figure out - "Figure out the solution."
* Work out - "Work out the answer."
* Derive - "Derive the formula."
* Prove - "Prove the theorem."
The best word to use will depend on the specific mathematical concept and the level of formality you're aiming for.