What Does it Mean to Derive a Formula?

It always super-confused me when I'd be reading the 5th edition of Macro Economics, and then there'd be this phrase of "deriving" or "solving" for the formula. I'd see a list of questions before hand so I was confused, where did the formula come from then??

Then I went on Google and looked this up: 

And then I read this:

That makes a lot more sense already!!!

"To derive a formula means to deduce, obtain, or prove the formula from a set of already-known or already-established principles or observations."

Steve Baker provided the best answer! 😀

Here is the link! :)