Create a math parser. Some suggestions were:

• Basic math parsing, if you type 5-(3-5)*6 then that would give 17.
• Draw functions like y = 5x + 2 or y = sqrt(25 - x^2)
• A program that can calculate 1 unknown x (or more but that would make it very hard); like 2x = 5 would give: x = 2.5