The velocity potential for a certain inviscid flow field is

$=-(9x^{2}y-y^3)$

where $$ has the units of $f\frac{t^2}{s}$ when $x$ and $y$ are in feet.

Determine the pressure difference $($in psi$)$ between the points $(1,2)$ and $(8,8),$ where the coordinates are in feet, if the fluid is water

and elevation changes are negligible.

$p_1-p_2=$

psi