$8{y}^{3}+4{y}^{2}-5y+12-\left( 3{y}^{2}-6y-18 \right)$
When there is a $-$ in front of an expression in parentheses, change the sign of each term of the expression and remove the parentheses$8{y}^{3}+4{y}^{2}-5y+12-3{y}^{2}+6y+18$
Collect like terms$8{y}^{3}+{y}^{2}-5y+12+6y+18$
Collect like terms$8{y}^{3}+{y}^{2}+y+12+18$
Add the numbers$8{y}^{3}+{y}^{2}+y+30$