Step-by-step explanation:
you do know that all angles at one side of a line around a single point have to be 180° (you can make only a half-circle on one side of a line).
therefore, y and 3y are supplementary angles (combined they are 180°).
y + 3y = 180
4y = 180
y = 45°
3y = 135°
and now again, the sum of all angles in a triangle is always 180°.
so,
180 = 135 + 18 + x
x = 27°
