Answer:
60
Step-by-step explanation:
The sum of two interior angles in a triangles is equal to one exterior angle that is supplementary to the third interior angle
3x+4x = 10x - 45
7x = 10x - 45 transfer 45 to the other side of equation and subtract 7x from 10 10
45 = 10x - 7x
45 = 3x divide both sides by 3
15 = x angle B is 4*15 = 60
