As can be seen from the figure, the tangent passes through (0,3), (4,5),

∴ The slope of the straight line L is 5? 34? 0= 12,

Because the derivative value of the curve at the tangent point is the slope of the tangent line of the curve,

So f'(4)= 12, and f (4) = 5.

Let g(x)=f(x)x, then g'(x)=xf'(x)? f(x)x2

So g ′ (4) = 4×12? 542=-3 16

So the answer is:? 3 16