1. **State the problem:** Simplify the expression $(-4s + 2t) - (-s + t)$. 2. **Recall the rule:** When subtracting a group, distribute the minus sign to each term inside the parentheses: $$a - (b + c) = a - b - c$$ 3. **Apply the rule:** $$(-4s + 2t) - (-s + t) = -4s + 2t + s - t$$ 4. **Combine like terms:** For the $s$ terms: $$-4s + s = (-4 + 1)s = -3s$$ For the $t$ terms: $$2t - t = (2 - 1)t = t$$ 5. **Write the simplified expression:** $$-3s + t$$ **Final answer:** $$\boxed{-3s + t}$$