The tangent line must pass through the point (4, 16a).
, so when x = 4 the slope of the tangent line is .
So the equation of the line that passes through the point (4, 16a) that has a slope of 8a is:
Insert the point (4, 16a) to find b:
So the tangent line is or in the form given in the problem:
Compare this with
What are your values for a and b?