a + c = (-2; -3; 6) + (8; -8; -1) = (6; -11; 5)
a + b + d = (-2; -3; 6) + (1; -2; 2) + (9; 9; -8) = (8; 4; 0)
a + b + c + d = (-2; -3; 6) + (1; -2; 2) + (8; -8; -1) + (9; 9; -8) = (16; -4; -1)
a + c = (-2; -3; 6) + (8; -8; -1) = (6; -11; 5)
a + b + d = (-2; -3; 6) + (1; -2; 2) + (9; 9; -8) = (8; 4; 0)
a + b + c + d = (-2; -3; 6) + (1; -2; 2) + (8; -8; -1) + (9; 9; -8) = (16; -4; -1)