Straight Lines And Pair Of Straight Lines

The line $2 x+y-3=0$ divides the line segment joining the points $A(1,2)$ and $B(-2,1)$ in the ratio $a: b$ at the point $C$. If the point $C$ divides the line segment joining the points $P\left(\frac{b}{3 a},-3\right)$ and $Q\left(-3,-\frac{b}{3 a}\right)$ in the ratio $p: q$, then $\frac{p}{q}+\frac{q}{p}=$