$\noindent I think they want you to assume that $\sqrt{x}$ is irrational. Then the fact that $a$ and $b$ are rational means that $a$ has to be $\frac{2}{3}$ and $b$ has to be $3$. This is based on the fact that if $\alpha$ is an irrational number and $a,a^\prime ,b,b^\prime$ are rational...