Horizontal and perpendicular tanget lines

Question

a. at what point(s) does the graph f(x) - x^3 - 3x have a horizontal tangent line?

b. at what point(s) does the graph y=x/x-4 have a tangent line perpendicular to y=1/4 x +1

Answer 1

a. At f'(x) = 0

b. At g'(x) = -4