# Question #c218d

Solving for $p$ means to "isolate" the variable, expressing it in terms of the others.
First of all, we observe that $4 p x + 9 p y = p \left(4 x + 9 y\right)$. This given, if we wanted to be particularly meticolous, we should observe that $\left(4 x + 9 y\right)$ must be non-zero, otherwise our equation would be $2 x = 0$, which can't be solved for $p$.
So, if our hypothesis holds, we can divide both sides by $\left(4 x + 9 y\right)$ (which is another reason why it should be non-zero), and obtain
$p = - \frac{2 x}{4 x + 9 y}$
which is exacly what we mean for "solving for $p$".