jmc

Let $d$ be the common difference. Then the last term is $7+(15-1)d=7+14d$. The sum of an arithmetic series is equal to the average of the first and last term, multiplied by the number of terms, so the sum of the series is $$\frac{7+(7+14d)}{2}\cdot 15=15(7d+7)=105d+105.$$We are told that this sum equals $-210$, so we have $105+105d=-210$, from which we find $d=\boxed{-3}$.

Note: $\boxed{3}$ is also accepted as an answer.