Find the stationary point by setting dy/dx=0. Observe that the expression is never negative. This means that at a point slightly to the left of the stationary point, the gradient is positive. At a point slightly to the right of the stationary point, the gradient is also positive. Since the shape is /‾‾/, the stationary point is therefore a point of inflexion. Since the x-coordinate is 0, it is also the y-intercept. To find the x-intercept, substitute y=0 into the equation. Plot these 2 points on the axes and join them suitably to obtain your cubic graph.