Find the Variation Equation for the Variation Statement $z$ Varies Jointly As

Multiple Choice

Find the variation equation for the variation statement.
- $z$ varies jointly as $y$ and the cube of $x ; z = 960$ when $x = 4$ and $y = - 5$

A) $y = 3 x ^ { 3 } y$
B) $y = - 3 x ^ { 3 } y$
C) $y = - 3 x y ^ { 3 }$
D) $y = 3 x y ^ { 3 }$