The units analysis tells you how to work this problem. In order to get from mg/min to mL/min, you need to multiply by a factor with units mL/mg. Those units are the inverse of the concentration in mg/mL, which is found from g/mL.
The problem statement suggests you should convert dimensions to inches to work this problem. .. 2.75 ft = 33 in .. 1.5 ft = 18 in The empty height is .. 0.6*7 in = 4.2 in so the empty volume is .. V = (33 in)*(18 in)*(4.2 in) = 2494.8 in³
Multiplying this by the conversion factor to gallons gives .. V = (2494.8 in)³ × (1 gal)/(231 in³) = 10.8 gal
It will take 10.8 gallons of gas to fill the tank.