Angle-5 and angle-7 are 'vertical angles', so they're equal,
and we can write ...
10x- 9 = 9x
Subtract 9x from each side: x - 9 = 0
Add 9 to each side: x = 9
Now that we know what 'x' is, we can find the size of Angles-5 and -7 .
Angle-7 = 9x = 81° .
Now look at Angle-6 ... the one that's the answer to the problem.
Angle-6 and -7 together make a straight line, so they must
add up to 180°.
Angle-6 + 81° = 180°
Subtract 81° from each side: Angle-6 = 99° .
