A trough 12 ft long has a cross section in the form of an isosceles triangle with base at top which is 4 ft deep and 6 ft across.

A trough 12 ft long has a cross section in the form of an isosceles triangle with base at top which is 4 ft deep and 6 ft across. If water is filling the trough at the rate of 9 ft3/min how fast is the water level rising when the water is 3 ft deep?