А) (1,2a-3,4b)+(-3,2a+-0,6b) б) -(2x+y)+(-6x-7y) в) (-5a^2-9a+1)-(-13a^2-9a+5) г)...

1)
$(1.2a - 3.4b) + ( - 3.2a + 0.6b) = \\ = 1.2a - 3.4b - 3.2a + 0.6b = \\ = - 2a - 2.8b$
1.1)
$(1.2a - 3.4b) + ( - 3.2a - 0.6b) = \\ = 1.2a - 3.4b - 3.2a - 0.6b = \\ = - 2a - 4b$
2)
$- (2x + y) + ( - 6x - 7y) = \\ = - 2x - y - 6x - 7y = \\ = - 8x - 8y$
3)
$( - 5 {a}^{2} - 9a + 1) - ( - 13 {a}^{2} - 9a + 5) = \\ = - 5 {a}^{2} - 9a + 1 + 13 {a}^{2} + 9a - 5 = \\ = 8 {a}^{2} - 4$
4)
$- (2 {x}^{2} - 3xy + 7) - ( - 2 {x}^{2} + 7xy - 9) = \\ = - 2 {x}^{2} + 3xy - 7 + 2 {x}^{2} - 7xy + 9 = \\ = 2 - 4xy$
5)
$( - 3a + b) - (b - 3a) = \\ = - 3a + b - b + 3a = 0$