Yep, this one seems sneaky and confusing. But it's not so bad if you remember the things you learned about parallel lines. (It can't be too tough ... I learned them in 1954 and I still know how to use them.)
Look at the picture. Line ' l ' is parallel to line ' m ', and the horizontal line on the bottom (which is not labeled) is a transversal that cuts the parallel lines.
Did you learn that interior angles on the same side of the transversal are equal ? I'm sure you did, although it may have a new name nowadays.
Anyway, with the help of that 'tool', angle-'B' and angle-'D' are equal. So . . .
