Differentiate following with respect to x.

(3x2 – 5ax + a2)4

Do you mean

f(x) = (3x^2 – 5ax + a^2)^4
?
Let u(x) = (3x^2 – 5ax + a^2)
f(x)= u^4
Then
df/dx = df/du * du/dx
= (4 u^3)*(6x -5a)
Multiply it out, using the definition of u(x).