I'm learning quadratic equations

I need practice solving them. I'm writing a spreadsheet so that I can get the correct answer.

I'll use this spreadsheet to make problems with answers for me to practice on

My spreadsheet can solve for x if a, b, and c are entered

I'd like to

solve a if x, b, and c are entered

and

solve b if x, a, and c are entered

and

solve c if x, a, and b are entered

The cells in purple are what I need

Edited

I just figured out that my spreadsheet has a bug.

I don't know how to take the square root of a negative number

The cell in question is p15

On the other stuff, it may not be possible. That's even better because my teacher won't ask these questions. (find c if x = 17)