Can someone explain how to do this? my books and don't didn't tell me what i'm supposed to do to solve this.

 x = 50



Explanation:

Let y be the missing interior angle of the quadrilateral.
So angle y is adjacent to angle x. 

Recall that the sum of all four angles of any quadrilateral is 360, so,
40+80+110+y = 360
230+y = 360
230+y-230 = 360-230 ... subtract 230 from both sides
y = 130



Angle y and angle x form a straight angle.
The two angles add to 180 degrees
x+y = 180
x+130 = 180
x+130-130 = 180-130 ... subtract 130 from both sides
x = 50

i believe it's a.

i'm not sure on this one, most likely correct.

good luck!