15. determine the values of the unknown variables.

i'm really just asking how i can get x. from there i can do it i'm just confused in this step

In a triangle, the three interior angles always add to 180°

Step 1  (ΔABD):
x+7 + 3x-18 + x-9 = 180
5x - 20 = 180
5x = 180 + 20
5x = 200
x = 40°

Step 2  (ΔBCD):
x+21 + x+17 + y = 180
2x + 38 + y = 180
2*40 + 38 + y = 180
80 + 38 + y = 180
118 + y = 180
y = 180 - 118
y = 62°

If you need an equation, it would be y=2x+8, y would be how long it grew, x being weeks.if you need how long he was asleep, it would be 4 weeks if he started with no beard, the equation would be x=8÷2if he did have a beard before hand, the equation would be 2w+x=8