Explanation:
2 things to remember and use here :
1. the sum of all angles in a triangle is always 180°
2. the sum of all angles around a singe point on one side of a line is also 180° (just think about it : if there is a circle with its center in that particular point like S here, then the line cuts the circle in half. and half a circle is 180°).
so now that we know all that, let's combine both principles :
the inner angle S = 180 - (12x + 4)
and the angle U is 3x +6, so
U = 3x + 6 = 180 - 70 - (180 - (12x + 4))
3x + 6 = 110 - 180 + (12x + 4) = -70 + 12x + 4 = -66 + 12x
72 = 9x
x = 8
S = 180 - (12×8 + 4) = 180 - 96 - 4 = 80°
U = 3x + 6 = 3×8 + 6 = 24 + 6 = 30°