I can't get beyond c = a x^2 + b x. How is this equation derived from the figure given? There's no explanation as to what "a" and "b" actually represent?

## Don't understand how you get to c = a x^2 + b x

I can't get beyond c = a x^2 + b x. How is this equation derived from the figure given? There's no explanation as to what "a" and "b" actually represent?