The density is in pounds per gallon, but the volume is in liters. So first let’s convert the 2.5 L to gallons. $$\begin{aligned} 2.5\,\text{L}\amp=\frac{2.5\,\text{L}}{1}\amp\amp\text{Rewrite as a ratio.}\\ \amp\approx\frac{2.5\,\text{L}}{1}\cdot\frac{1\,\text{gal}}{3.785\,\text{L}}\amp\amp1\,\text{gal is about }3.785\,\text{L.}\\ \amp=\frac{2.5\,\cancel{\text{L}}}{1}\cdot\frac{1\,\text{gal}}{3.785\,\cancel{\text{L}}}\amp\amp\text{Units may now cancel.}\\ \amp=\frac{2.5}{1}\cdot\frac{1\,\text{gal}}{3.785}\amp\amp\text{Multiply what's left.}\\ \amp=\frac{2.5}{3.785}\,\text{gal}\amp\amp\text{Divide.}\\ \amp\approx0.6605\,\text{gal} \end{aligned}$$ Now we can multiply the density (6.9 lb/gal) by the volume ($\approx0.6605\,\text{gal}$). We find $6.9\cdot0.6605\approx4.557\text{,}$ so the oil weighs about $4.557$ pounds. With units: $$\begin{aligned} 6.9\,\frac{\text{lb}}{\text{gal}}\cdot0.6605\,\text{gal}\amp=\frac{6.9\,\text{lb}}{1\,\text{gal}}\cdot\frac{0.6605\,\text{gal}}{1}\\ \amp=\frac{6.9\,\text{lb}}{1\,\cancel{\text{gal}}}\cdot\frac{0.6605\,\cancel{\text{gal}}}{1}\\ \amp=6.9\cdot0.6605\,\text{lb}\\ \amp\approx4.557\,\text{lb} \end{aligned}$$