Answer:
1.
= 2x + 3y + 5x + 6y \\ = 2x + 5x + 6y + 3y \\ = 7x + 9y
2.
= 7 - 3a + 5a + 15 \\ = - 3a + 5a + 15 + 7 \\ = 2a + 22
3.
= fg - 5fg - g + 3f \\ = - 4fg + 3f - g
4.
= 6 {lm}^{2} - {5l}^{2} m - {17lm}^{2} + {21l}^{2} m \\ = 6 {lm}^{2} - {17lm}^{2} - 5 {l}^{2} m + {21l}^{2} m \\ = - 11 {lm}^{2} + 16 {l}^{2} m
5.
= 50xy - 55x {y}^{2} + 63xy + {70xy}^{2} \\ = - 55 {xy}^{2} + {70xy}^{2} + 50xy + 63xy \\ = 15 {xy}^{2} + 113xy
