Explanation:
42=3v-15
We move all terms to the left:
42-(3v-15)=0
We get rid of parentheses
-3v+15+42=0
We add all the numbers together, and all the variables
-3v+57=0
We move all terms containing v to the left, all other terms to the right
-3v=-57
v=-57/-3
v=+19