First of all, solve the second parenthesis: if we raise a product to a certain power, it is the same as raising each factor to that power:
[tex] (4x)^3 = 4^3\cdot x^3 = 64x^3 [/tex]
So, the expression becomes
[tex] 5x^3\cdot 64x^3 [/tex]
Now, we multiply the numeric parts:
[tex] 5\cdot 64 = 320 [/tex]
and the literal part:
[tex] x^3\cdot x^3 = x^{3+3} = x^6 [/tex]
So, the answer is [tex] 320x^6 [/tex]
