Hi MG,
It is not rude to ask for explanation.
I am very pleased that you are learning from me. That is what I am here for. - to teach people 
Plus intelligent, enthusiastic students like you are fun and rewarding to teach. 
$$y=8\times \left(\frac{2}{4}-\frac{\frac{6^3}{2}}{15}\right)$$
\frac{\frac{6^3}{2}}{15}
$$\frac{\frac{6^3}{2}}{15}$$
I have 'nested' a fraction inside another fraction, that is why there are two \frac
The black parentheses belongs to the main fraction. The red parentheses belong to the nested fraction.