$$(\frac{y}{a}+5a)[(\frac{y}{a})^{2}-\frac{y}{a}+5a+(5a)^{2}]$$
$\frac{\left(y+5a^{2}\right)\left(y^{2}-ay+25a^{4}+5a^{3}\right)}{a^{3}}$
$\frac{y^{3}-ay^{2}+25ya^{4}+5\left(ay\right)^{2}+125a^{6}+25a^{5}}{a^{3}}$
