z varies jointly with y and the square of x. If z=256 when

Question

y=−8 and x=−4, find z when x=−3 and y=−4.

Answer 1

"z varies jointly with y and the square of x" ----> z = k(y x^2)
256 = k(-8(16))
k = -2

so you know z = -2yx^2

plug in your given values and find z