Distribute the 5 to both b and 3
5(b) = 5b
5(3) = 15
5b + 15 = b + 39
move all the b's to one side and the numbers to the other (do the opposite sign)
5b (-b) + 15 (-15) = b (-b) + 39 (-15)
5b (-b) = 39 (-15)
4b = 24
divide 4 from both sides
4b/4 = 24/4
b = 24/4
b = 6
6 is b, and is your answer
hope this helps