1). 15a + 7b - 5c - 11a - 12b + 13c

= 15a - 11a + 7b - 12b - 5c + 13c

= (15 - 11)a + (7 - 12)b + (-5 + 13)c

= 4a - 5b + 8c

2). -3m + 4n - 6 + 7n - 8m + 10

= -3m - 8m + 4n + 7n - 6 + 10

= -11m + 11n + 4

3). 16a - 12b + 4 + 5a - 9b + 2c

= 16a + 5a - 12b - 9b + 2c + 4

= 21a - 21b + 2c + 4

4). (10p + 11) - (2p - 5)

= 10p + 11 - 2p + 5

= 10p - 2p + 11 + 5

= 8p + 16

5). (6 + 5n) - (5 - 3n)

= 6 + 5n - 5 + 3n

= 5n + 3n + 6 - 5

= 8n + 1

6). 7(x - 3) = 7(x) + 7(-3) = 7x - 21

7). (a + 3)(a + 5)

= a(a) + a(5) + 3(a) + 3(5)

= a² + 5a + 3a + 15

= a² + 8a + 15

8). (b + 6)(b - 2)

= b(b) + b(-2) + 6(b) + 6(-2)

= b² - 2b + 6b - 12

= b² + 4b - 12

9). (c - 3)(c + 4)

= c² + 4c - 3c - 12

= c² + c - 12

10). (2x - 1)(x - 2)

= 2x² - 4x - x + 2

= 2x² - 5x + 2