The answer would be 140.
Using knowledge that a straight line adds up to 180 degrees, to find the missing angle between 80 and 50 would be 50. ( 80+50 gives you 130. 180-130 gives you 50)
Now do 50 + 30 as those are the only know angles in the quadrilateral which gives you 80. Quadrilaterals add up to 360 so do 360-80 to find out the value of x + x . This give you 280. Now you divide 280 by 2 to give you 140. That how you work it out.
Hope this really helps.