Angles on a straight line always adds up to 180°
The angle on the left side of 100° is, (180-100) = 80
Do the same with 80° and the angle beneath it (Remember! angles add up to 180° on a straight line).
Hence, 180 - (x+105) = 80
=> 180-80 = x+105
=> 100-105 = x
x = -5
