y = x3
dy/dx = 3x2
For the line y = mx + b to be a tangent we require
3x2 = 3
x = -1, 1
Sub this into the curve y = x3 then we get (-1, -1) and (1, 1) as the points of contact.
Since y = 3x + b is a tangent then substituting each of those points allows you to solve for multiple values of b.