How to apply Arden's theory to determine a regular expression
Arden's theorem states that $A^*\,B$ is the least fixed point of the equation:
$$ X = A\,X\,\cup B$$
and that $A\,B^*$ is the least fixed point of the equation:
$$X = X\,B\,\cup A$$
In your case, $R = Q\,P^* = a^* (a b)^*$.