I agree with the answer below. Once you have the freebody diagram drawn (in this case it sounds like you should draw one *for the elevator* since it asks for the tension in the cable **where it is attached to the elevator**), use it to determine a NET FORCE expression (In a problem like you described, it should be something like: Tension - Force of Gravity. Where Force of Gravity = mg = 900 x 9.8 assuming this elevator is on Earth). Once you have that expression then plug into Newton's Second Law: a = Net Force/mass or 2.5 = (Tension - 900 x 9.8)/900 then solve for Tension.
One final note, It is important to pay attention to the signs of anything that is a vector (all of the forces and the acceleration) to be sure anything pointing in the same direction is given the same sign. In this case, acceleration and tension both point UP and so I made them both positive, but the force of gravity points DOWN and so it is negative.