I am trying to find the maximum velocity of the flow using the equation below. However, I cannot come across the correct derivation (which I must set equal to 0 to find my maximum velocity). I keep getting an x in my final answer....

$\displaystyle v_{z}(x)= \frac{\rho g\delta^{2} }{2\mu }[1-(\frac{x}{\delta })^{2}]$