z varies jointly with y and the square of x. If x=-2 when y=7 and z=-84,find x when z=-96 and y=2

Question

I just need help figuring out what equation to use.

Answer 1

z = kyx^2
so,
z/(yx^2) = k is constant. So, you want x such that

-96/(2x^2) = -84/(7*4)

Answer 2

Z varies jointly as x and square root of y