Check the picture, notice the domain in red.
recall that the domain is the values for the x-coordinate, whilst the range is the values for the y-coordinate.
2x + 1.5x +20 = 90
3.5 x = 90-20
3.5 x = 70
x = 70/3.5
x = 20
angle DAE = 2*20 = 40
angle CAD = 1.5*20 +20 = 50