I am having trouble understanding how or even where to start proving that the possible group is associative.

14: Given the groups $\mathbb{R}*$ and $\mathbb{Z}$. Let $G=\mathbb{R}^* \times \mathbb{Z}$. Define a binary operation $\circ$ on $G$ by $(a,m) \circ (b,n) = (ab, m+n)$. Show that $G$ is a group under this operation.