(a)x+y = 180; y = 180-x;
when x = 30, y = 180-30 = 150
when x = 60, y = 180-60 = 120
when x = 90, y = 180-90 = 90
when x = 120, y = 180-120 = 60
when x = 150, y = 180-150 = 30
when x = 180, y = 180-180 = 0

(b)xy+3x+6y+18; x(y+3)+6(y+3); (x+6)(y+3)

(c)(3+x)(5-2x) = 3(5-2x)+x(5-2x); 15-6x+5x-2x² 15-x-2x²