3.79: Binomial expansion of $\left(2 + \dfrac{x}{2}\right)^{n}$

In the binomial expansion of $\left(2 + \dfrac{x}{2}\right)^{n}$, the first three terms in increasing powers of $x$ are $b + abx + abx^{2}$. Find the values of the constants $n$, $a$ and $b$.