you want to get all the numbers on the right side by adding the opposite of what is on the left side, to both sides:
6 + -6 +3b =-18 +-6
3b =-24 after simplifying the numbers on both sides
now make the coefficient of the variable "1" by multiplying both sides by the reciprocal of the coefficient that is there:
3(1/3)b =-24(1/3)
b=-8