If you lift a 45 lb. rock 3 ft. from the ground, how much work have you done?

Work is force times distance. In this case, the force is the weight of the rock so:
\(W=f \times d=45 \mathrm{lb.}\times 3 \mathrm{ft.}=135 \mathrm{ft} . \mathrm{lb} .\)