We can use the power rule to take the first derivative of the function, set it = 0, and solve using quadratic formula or factoring (since the first derivative of a cubic function will be quadratic):

f^{'}(x) = 6x^{2} + 6x - 120 = 0 (since f^{'} gives us the slopes of tangents to f, and the slope of a horizontal line = 0)

x^{2} + x - 20 = 0 (dividing both sides by 6)

(x - 4) (x + 5) = 0

x = 4 or x = -5