How do I get to improve this code?

Question

How can I design this better. I'll also appreciate if you help me improve the calculator.

https://code.sololearn.com/WEx3a2gRsQ6m/?ref=app

Answer 1

Are there functions to be added?

Answer 2

It look better if it was centered
It be easier to read / add on if you used js instead of the form tag imo

Answer 3

I don't actually know js. But I'm gonna start on it.