The area of a circle in terms of the circle's radius

is

Differentiating both sides with respect to

, you get
Y=mx+c isn't an equation to solve. It's the slope-intercept formula for graphing lines.
y and x are the variables, and m and c are the constants.
As an example, the line y=4x+2 looks like this:
6x-3
You have to put f(x) into g(x) to find the answer, so you rewrite it like this: 3(2x-1)