Algebra

$a+ar+ar^2+...=\frac{a}{1-r}$ as long as r<1

$a^3+a^3(r^3)+a^3(r^3)^2+...=\frac{a^3}{1-r^3}$  as long as r^3<1

So:  

$\frac{a}{1-r}=4$

and

$\frac{a^3}{1-r^3}=10$

Can you take it from here?