1. The vectors $a$ and $b$ are defined by

$a=pqi-24j+9rk$

$b=6i+(p-r)j+(p-3q)k$

where $p$, $q$ and $r$ are scalar constants.

Given that $a=3b$, and that $r>0$, find the value of $p$, the value of $q$ and the value of $r$.