Answer:
129
Step-by-step explanation:
The shape shown is a parallelogram, which means that adjacent angles are supplementary. Adjacent angles are angles that share rays; this means that <A is adjacent to both <B and <D. Supplementary angles are angles whose sum is 180. Therefore, to find <B subtract 51 from 180. 180-51=129. So, angle B (and D) must be 129.
