To solve this equation, we first simplify both sides:

8у - 9 - (4у - 5) = 12у - (4 + 5у)

8у - 9 - 4у + 5 = 12у - 4 - 5у

Combine like terms:

4у - 4 = 7у - 4

Subtract 4у from both sides:

-4 = 3у - 4

Add 4 to both sides:

0 = 3у

Divide by 3:

у = 0

Therefore, the solution to the equation is у = 0.