Barrons AP Calculus - David Bock

(E) The slope of y = x^3 is 3x^2. It is equal to 3 when x = ±1. At x = 1, the equation of the
tangent is
y − 1 = 3(x − 1) or y = 3x + 2.
At x = −1, the equation is
y + 1 = 3(x + 1) or y = 3x + 2.