doprowadz do najprostrzej postaci

a) (-5a^2 + 6a - 12) + (7a^2 - 11a + 9) =
b) (13x^2 - 15x + 9) - (19x^2 - 7x + 12) =
c) -9 (5a + 7b - 3) =
d) 4a^2 (-3a^3 + 6a^2 - 8) =
e) 3 ( - 2 a^2 + 5 a - 7 ) - 4 ( 6 a^2 + 4 a - 3 ) =
f) y (4 x y - 11 y^2) + 5 y^2 (-2x + 3 y) =