1) 4x - 3y - 2x + 9y = 4x - 2x + 9y - 3y = 2x + 6y;

2) 5(3x - 2) - 6 = 15x - 10 - 6 = 15x - 16;

3) 18a + (a - 4) - (15a + 3) = 18a + a - 4 - 15a + 3 = 4a - 1.