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
So x = 114 degrees
