Question

A plumb bob is a weight hung at the end of a string, called a plumb line. The weight pulls the string down so that the plumb line is perfectly vertical. Suppose that the angle formed by the wall and the roof is 123° and the angle formed by the plumb line and the roof is 123°. How does this show that the wall is perfectly vertical?

Could you guys help me my homework please?
Steve
remember your geometry, and theorems about angles formed by transversals of parallel lines, and their converses.