An explanation for this is that a straight line has a angle degree value of 180. Therefore if you draw the line that creates a 139 degree angle, the rest of the line would have an angle measurement of what is left of the 180 degrees. So to get x you would start with the 180 degree straight line value and subtract the 139 degrees of the line that divides it. so equation is:
180 - 139 = x
And and you get 41 degrees.
(To word this another way the two angles of x and 139 have to add up to the straight line of 180 degrees. Therefore another equation could be:
139 + x = 180
which is basically the same equation but might be easier explanation for some to understand.)
questions 7 and 8 are the same but instead of straight line with 180 degrees, you start with right angle with 90 degrees. Therefore x and the 18 degree angle have to add up to 90 degrees making an equation of:
x + 18 = 90
Or
x = 90 - 18
answer would be
x = 7w