C varies directly as the cube of a and inversely as the 4th power of B

C varies directly as the cube of a and inversely as the 4th power of B

The cube of a means we raise a to the 3rd power:
a^3

The 4th power of B means we raise b to the 4th power:
b^4

Varies directly means there exists a constant k such that:
C = ka^3

Also, varies inversely means we divide by the 4th power of B
C = ka^3/b^4

Varies directly as means we multiply by the constant k.
Varies inversely means we divide k by the term which has inverse variation.