Suppose the line y = 5x - 4 is tangent to the curve y = f(x) when x = 3. If Newton's method is used to locate a root of the equation f(x) = 0 and the initial approximation is x1 = 3, find the second approximation.

I am not sure how to find f(x). I tried finding the anti-derivative of the tangent line and used that as my f(x), but that didnt work.