The sum of the angles of a triangle is 180. We are given the measures of the two remote angles (ones furthest from x), so we can find the third angle by subtracting the given angles from 180:
180 - 67 - 20 = 93
Now that we have the supplementary angle to x, we can find x by subtracting 93 from 180. This is because supplementary angles must add up to 180. We know 93 and x are supplementary because if we remove the line that separates them, we are left with a straight line.
180 - 93 = 87
x = 87
