Please could someone help me the way to approach this?
Not quite! You left out the binomial coefficient that was already in inayat's formula.
The binomial theorem says that
$(a+b)^n = \sum \limits_{k=0}^n \begin{pmatrix}n \\ k\end{pmatrix} ~a^k ~b^{n-k}$
Where $\begin{pmatrix}n \\ k\end{pmatrix}$ is the "binomial coefficient", $\begin{pmatrix}n \\ k\end{pmatrix}= \frac{n!}{k!(n-k)!}$.