If you look from the points at angle 73, to 42, to 131, to z, the shape makes a quadrilateral. Since the total of the interior angles in a quadrilateral add up to 360 you make an equation:
360 = 131 + 73 + 42 + 7 + x Re-arrange the equation to get the x alone, by subtracting: 360 - 131 - 73 - 42 = x 114 = x
