Find the Variation Equation for the Variation Statement $\mathrm { z }$

Multiple Choice

Find the variation equation for the variation statement.
- $\mathrm { z }$ varies jointly as $\mathrm { y }$ and the cube of $\mathrm { x } ; \mathrm { z } = 1792$ when $\mathrm { x } = 4$ and $\mathrm { y } = - 4$

A) $y = - 7 x ^ { 3 } y$
B) $y = - 7 x y ^ { 3 }$
C) $y = 7 x y ^ { 3 }$
D) $y = 7 x ^ { 3 } y$