If y varies directly as square of x and the square root of z and y=3 when x=4 and z=9,find the value of y when x=3 and z=16

y = k x^2 √z

Or,
y/(x^2 √z) = k and is constant. So,

3/(4^2 √9) = y/(3^2 √16)

Now just solve for y.