Z varies jointly as the 4th power of x and the 5th power of y

Z varies jointly as the 4th power of x and the 5th power of y

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

The 5th power of y means we raise y to the power of 5:
y^5

The phrase varies jointly means we have a constant k such that:
z = kx^4y^5