jmc

Let us plug in $(5\ast x)=1$ into $3a-b$. Thus it would be $3(5)-x$. Now we have $2\ast (15-x)=1$. Plugging $2\ast (15-x)$ into $3a-b$, we have $6-15+x=1$. Solving for $x$ we have $$-9+x=1$$$$x=\boxed{10}$$