The answer is: mass is 40.17 kilograms.
d = 0.758 g/mL; density of fuel.
V = 14.0 gal; volume.
A gallon is a unit of volume in both the US customary and imperial systems of measurement. The US gallon is defined as 231 cubic inches (3.785 liters).
1 gal = 3785.41 mL.
V = 14 gal · 3785.41 mL:
V = 52995.74 mL.
m = 52995.74 mL · 0.758 g/mL.
m = 40170.77 g; mass of fuel.
m = 40170.77 g ÷ 1000 g/kg.
m = 40.17 kg.