Differentiation-rate of change

A point P(x,y) moves along the curve y = x e^(-x/4)
in the first quadrant. Its x-coordinate moves at the rate 2 units/s to the right. Perpendiculars are dropped from P to both axes so as to form a rectangle with the axes. When x = 4,
(a) what is the rate of change of the y-coordinate of P?
(b) how fast is the area of the rectangle changing?