1. **State the problem:** Simplify the expression $y - 4m - 3y - 5m$. 2. **Group like terms:** Combine the terms with $y$ and the terms with $m$ separately. $$y - 3y - 4m - 5m$$ 3. **Simplify the $y$ terms:** $$y - 3y = \cancel{1}y - \cancel{3}y = -2y$$ 4. **Simplify the $m$ terms:** $$-4m - 5m = -\cancel{4}m - \cancel{5}m = -9m$$ 5. **Write the simplified expression:** $$-2y - 9m$$ **Final answer:** $-2y - 9m$