Since we know A and B are alternate interior angles, they have to be equal because of the theorem of alternate interior angles, so we just have to equal the two equations:
4x+18=x+48
4x-x=48-18
3x=30
x=30/3
x=10
Angle B= x+48, then substituing x=10
Angle B would be= 10+48
Angle B=58 degrees