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.
