1. **State the problem:** Simplify the expression $$(3.4a - 1.7b) + (2.5a - 3.9b)$$. 2. **Use the distributive property and combine like terms:** $$3.4a - 1.7b + 2.5a - 3.9b$$ 3. **Group like terms:** $$ (3.4a + 2.5a) + (-1.7b - 3.9b) $$ 4. **Add the coefficients of like terms:** $$ 3.4a + 2.5a = 5.9a $$ $$ -1.7b - 3.9b = -5.6b $$ 5. **Write the simplified expression:** $$ 5.9a - 5.6b $$ **Final answer:** $$5.9a - 5.6b$$