Simplify the algebraic expression. 3) $(8 y+12)-(2 y-9)$

Step 1 ：Distribute the negative sign to the terms inside the parentheses: $(8y + 12) - (2y - 9) = 8y + 12 - 2y + 9$

Step 2 ：Combine like terms: $8y - 2y + 12 + 9 = 6y + 21$

Step 3 ：So, the simplified form of the given algebraic expression is \(\boxed{6y + 21}\)