jmc

Since the cube's volume is $5x$ cubic units, each side measures $\sqrt[3]{5x}$ units. The surface area is then $6(\sqrt[3]{5x}{)}^2$. We are told the surface area is also $x$. We have the equation $6(\sqrt[3]{5x}{)}^2=x$ Solving for $x$, we find that $x=\boxed{5400}$.