n our text book the following equation is given:

Question

P=(pm - p)g * R
where P is the pressure drop
R is the manometer reading
pm is the density of the manometer fluid
p is the density of the process fluid in the pipe
g is the acceleration do due to gravity
Then our professor told us this:
It is known that the flow rate measured by the orifice plate / manometer system is proportional to the square-root of the measured pressure difference. If the flow rate of the fluid is 123 gpm (gallons per minute) when the manometer reading is 32 cm, what is the maximum flow rate that can be measured by this manometer?
I have no idea how to solve for this please help.