This paper provides the formula for the elevation angle at which a projectile has to be fired in a vacuum from a general position to hit a target at a given distance. A spreadsheet application that models the trajectory is presented, and the problem of finding the points of shot and impact of a projectile moving in a vacuum if three points of the trajectory are known is solved and modelled by another interactive application. A Gaussian elimination method is used to solve the system of linear equations that gives the trajectory parameters. These applications were developed with 30 high-school students, who were given a questionnaire to find out their opinion of the lessons.