Question

find f'(x)
find the slope of the graph of f a x=2 and x=4
find the equations of the tangent lines at x=2 and x=4
and find the value(s) of x where the tangent line is horizontal.

f(x)=2x^2+8x

I found that f'(x)=4x+8
and that the slopes were 16 and 24
and the equations were y=16x-8 and y=24x-46 but i couldnt find out how to do the last part.

Answer 1

horizontal means zero slope

set f' equal to zero and solve for x

Answer 2

what, you can't solve

4x+8 = 0

??