Find the gradient of the curve $y = x^3 - 3x$ at the point where $x = 2$.

The gradient function is $\dfrac{dy}{dx} = 3x^2 - 3$. At $x = 2$ this is $3(4) - 3 = 9$. Substituting into $y$ rather than $\dfrac{dy}{dx}$ gives $y = 2$, which is the value of the function, not the gradient.