You don't "divide by the square" - c ² means you are multiplying c by c.
To solve for c in this equation, you would have to do the opposite of taking a square, which in this case means taking the square root of both sides.
c ² = 80
==> √(c ²) = √80
==> |c| = √80
Then c = +√80 or c = -√80.