\(a,=7\left(x-y\right)+4a\left(x-y\right)-5\\ =\left(7+4a\right)\left(x-y\right)-5\\ =\left(7+4a\right).0-5=-5\\ b,=ab\left(x-y\right)+121\left(x-y\right)+x+123\\ =\left(ab+121\right)\left(x-y\right)+x+123=\left(ab+121\right).0+x+124\\ =0+x+123\\ =x+123\)