To calculate the average force due to air resistance on a projectile with a mass of 0.750 kg and an initial velocity of 18.0 m/s that reaches a maximum height of 11.8 m, one must consider the energy lost to air resistance. The initial kinetic energy can be determined using the mass and initial velocity, while the final potential energy is derived from the maximum height. The difference between these energy values represents the work done by air resistance. If air resistance were absent, the projectile would reach a higher altitude, which can also be calculated. Understanding these energy transformations is crucial for determining the average force due to air resistance.