Using Mean Value Theorem to show Taylor Series expansion at second derivative

Question

$l(a) = l(0) + l'(0)a + \frac{1}{2}l''(\alpha)a^2$ where $\alpha\in [0, a]$

I understand how this works for the first derivative (simply move $l(0)$ to the lhs), but I'm not seeing it for the second derivative component. Thanks!