please help im not understanding please check this before skippping over it

Answer:
36degrees
Step-by-step explanation:
2x + 26 = 3x + 21
2x - 3x = 21 - 26
-x = -5
x = 5
m<B = (3x+21)°
m<B = 3(5) + 21
m<B = 15+21
m<B = 36degrees