y varies directly as x and inversely as the square of z means

y = k*(x/z^2), where k is constant

First we will find k using the fact that y=6 when x=50 and z=5

6 = k*(50/5^2)

k = 3

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y = 3(x/z^2)

for x = 64 and z = 4 we have

y = 3(64/4^2)

y = 3(64/16)

y = 3*4

y = 12.