What else could you call the least common multiple in an addition or subtraction problem invloving fractions

Pls! does anybody know a shortcut for answering these types of questions in the future?

Which equation results from adding the equations in this system? x+6y=9 and -x+2y=-15

The population of a small rural town in the year 2006 was 2,459. the population can be modeled by the function below, where f(x residents and t is the number of years elapsed since 2006. f(t) = 2,459(0.92)