Since vertical angles are congruent, the measure of angle A would be equal to the measure of angle B.
3x + 1 = 4x - 21 step 1 - subtract 3x from each side
1 = x - 21 step 2 - add 21 to each side
x = 22 step 3 - simply solve for x
Now, to find the measure of angle B, just plug in x.
4x - 21 step 1 - write angle B's equation
4(22) -21 step 2 - plug in x, or 22
88 - 21 = 67 step 3 - simplify the expression
Therefore, the measure of angle B is 67 degrees.