Answer:
[tex]m\angle B=109^o[/tex]
Step-by-step explanation:
we know that
Quadrilateral ABCD is a trapezoid
A trapezoid has two parallel lines
Segment AD is parallel to segment BC
so
[tex]m\angle A+m\angle B=180^o[/tex] ----> by consecutive interior angles (supplementary angles)
we have
[tex]m\angle A=71^o[/tex]
substitute
[tex]71^o+m\angle B=180^o[/tex]
[tex]m\angle B=180^o-71^o[/tex]
[tex]m\angle B=109^o[/tex]
