A tiny uncertainty on estimation

Problem :

If the amount of the first $p$ regards to a math development is $q$ and also the amount of the first $q$ terms is $p$, after that locate the amount of $p+q$ terms.

For the trouble we can write (taking into consideration $a$ is the first term and also $d$ is the usual distinction) :

$$\frac{p}{2}\cdot \biggl[2a + (p-1)d \biggr] = q \qquad \cdots (1)$$

$$\frac{q}{2}\cdot \biggl[2a + (q-1)d \biggr] = p \qquad \cdots (2)$$

Now in my component it is considered that from these we can write $\displaystyle d= \frac{-2(p+q)}{pq}$ ; I am not obtaining just how we can get that value of $d$?!