In an equation, the phrase "three times the sum of y and 8 is 51" means: 3(y + 8) = 51.
First, distribute 3(y + 8) by multiplying the 3 into the parentheses. You are left with 3y + 24 = 51. To simplify further, subtract 24 from both sides because both do not have variables and you need to get rid of the 24 on the left.
3y + 24 - 24 = 51 - 24
3y = 51
Now, divide by 3 on both sides to leave y by itself.
3y / 3 = 51 / 3
y = 17