Do you need to add, in addition to what you also calculated, the force from Newton's Second Law, F = ma? So net force would be mg + ma, factoring into m(g + a). This would be a case of just finding net acceleration, which is the acceleration due to gravity plus any other acceleration. Also, not sure how much explaining you need (I'm probably saying stuff you already know), but you have to make sure you follow convention when finding net acceleration, as acceleration is a vector. For instance, if the elevator were accelerating *downwards* at 2.0 m/s squared, the net acceleration would be g - a, because the acceleration of the elevator is considered negative.

Hope this helps!