Assuming 'a' is independent of x, since nothing else is given.

`y' = d/dx (a^3) + d/dx(cos^3(x))`

`= 0 + 3*cos^2(x)*d/dx(cos(x))`

`= -3 sin(x) cos^2(x)`

hope this helps

Assuming 'a' is independent of x, since nothing else is given.

`y' = d/dx (a^3) + d/dx(cos^3(x))`

`= 0 + 3*cos^2(x)*d/dx(cos(x))`

`= -3 sin(x) cos^2(x)`

hope this helps