Explanation:
2 things to remember :
the sum of all angles in a triangle is already 180°.
the inner and the outer angles at a corner point of a triangle are always supplementary angles (a linear pair) meaning that together they are 180°.
so, the inner angle at R is
R = 180 - 140 = 40°
180 = 40 + (8x + 4) + (3x + 4) = 40 + 11x + 8 =
= 48 + 11x
11x = 132
x = 132/11 = 12
so, the angle S is
S = 3×12 + 4 = 36 + 4 = 40°