2. а) 6 + 4a - 5a + a - 7a; б) 5(n - 2) - 6(n + 3) - 3(2n - 9); в) 5/7 * (2.8c - 4 1/5 d) - 2.4 * (5/6 c - 1.5d).

2. Упростите выражение:

1. \( 6 + 4a - 5a + a - 7a = 6 + (4 - 5 + 1 - 7)a = 6 - 7a \)
2. \( 5(n - 2) - 6(n + 3) - 3(2n - 9) = 5n - 10 - 6n - 18 - 6n + 27 = (5 - 6 - 6)n + (-10 - 18 + 27) = -7n - 1 \)
3. \( \frac{5}{7} \left(2.8c - 4\frac{1}{5}d\right) - 2.4 \left(\frac{5}{6}c - 1.5d\right) = \frac{5}{7} (2.8c - 4.2d) - 2.4 \cdot \frac{5}{6}c + 2.4 \cdot 1.5d = \frac{5}{7} \cdot 2.8c - \frac{5}{7} \cdot 4.2d - 2c + 3.6d = 2c - 3d - 2c + 3.6d = (2 - 2)c + (-3 + 3.6)d = 0.6d \)

Ответ: а) 6 - 7a; б) -7n - 1; в) 0.6d.