Seems easy but need help

Question 3 (1 point) Determine what value(s) of x is the slope of the tangent to f (x ) = = xi- # x3 - 8 horizontal. a) x = 0 and x= -4 b) x = 4 and x = -4 O c) x = -2 and x = 4 Od) x = 0 and x = 3