Angles and Lines
We are given a figure where lines l and m are parallel and they are crossed by a third line.
Adjacent angles in the point where the lines are crossed are called linear pairs because they add up to 180°. Since both given expressions correspond to linear angles, it follows that:
9x - 4 + 5x + 16 = 180
Simplifying:
14x + 12 = 180
Subtracting 12:
14x = 180 - 12 = 168
Dividing by 14:
x = 168/14
x = 12