If you want to DIY it, then a basic Arduino Uno and a touchscreen shield are all that's needed for hardware -- no soldering either.

Here's a similar project, showing what it might look like. They've already done the programming for a "keypad" and "result display" on the touch screen. So you'd just have to remove any code that has to do with the telephone shield part.

https://learn.adafruit.com/arduin-o-phone-arduino-powered-diy-cellphone

Or here's an even better one: A calculator. You simply have to customize the code with your simple formula:

https://www.instructables.com/Arduino-TFT-LCD-Touchscreen-Calculator/

Cheers