sequences and series - Find the first term of an arithmetic progression, given the 7th and the 16th terms

I tried to solve this problem by first finding out the common difference by using the formula
$$
\text{common difference} = \frac{T_p-T_q}{p-q}
$$
with $T_7=-1$ and $T_{16}=17$. But now I'm not able to find the first term. I tried out many methods. Please help me out..