First we need to assume that the sum of the angles is also 180°. if one is less than 90°, the other will be more. we can do this because in a plane 4-sided polygon the sum of the inner angles will always be 360°, and we already know the two angles on the right side
so we can say that:
3x + 20 + x = 180
Since we got only one variable in this equation, we can already solve for x.
4x + 20 = 180
4x = 160
x = 40
the lower left angle is just 40°
the upper left is three times that plus 20, so it's 160°
hope this helps you to get the method