The two little lines through the sides of AB and CB mean that they are the same length.
Because the two sides are the same length angle A and Angle C are also the same, so those are both 73 degrees.
The three inside angles of a triangle need to equal 180 degrees, so we know angle B would equal 180 - 73 - 73 = 34 degrees.
Now we can set the equation for angle B equal to 34 and solve for x.
6x+4 = 34
Subtract 4 from each side:
6x = 30
Divide both sides by 6:
x = 30/6
x = 5