Answer:
[tex]\huge \boxed{\mathrm{129\° }}[/tex]
Step-by-step explanation:
Adjacent angles in a parallelogram add up to 180 degrees.
[tex]\mathrm{m \angle A + m \angle B=180}[/tex]
Solving for [tex]\mathrm{m \angle B}[/tex].
[tex]\mathrm{m \angle B=180-m \angle A}[/tex]
[tex]\mathrm{m \angle B}=180-51[/tex]
[tex]\mathrm{m \angle B=129}[/tex]
