On the number line bug A is moving in the positive direction from the point 0, and a bug B is moving in the negative direction from the point 0. The speeds of both bugs are constant. Both leave 0 at the same time, and three seconds later A has reached 18 and B has reached -15 If x is the point reached by A after t seconds, and y is the point reached by B after t seconds, express x and y in terms of t.