Hey Cake.

Are you assuming your shape is a parallelogram (or a rhombus)? If so then you can use the fact that angle A = angle C and angle B = angle D where total sum of angles is 360.

So basically you have 2*angle(A) + 2*angle(B) = 360 where

2x/3 = x/2 + 20

4x = 3x + 120 (multiply everything by 6)

x = 120

This implies angle(A) = angle(C) = 2*120/3 = 80.

Since 2*angle(A) + 2*angle(B) = 360 you have angle(B) = (360 - 2*80)/2 = angle(D) = 100.