a. 3x - 3 + 5(x - 1)
= 3(x - 1) + 5(x - 1)
= (3 + 5)(x - 1)
= 8(x - 1)
b. x2 - 25 + y2 - 2xy
= (x2 - 2xy + y2) - 25
= (x - y)2 - 52
= (x - y + 5)(x - y - 5)
c. x2 + 2xy - 16a2 + y2
= (x2 + 2xy + y2) - 16a2
= (x + y)2 - (4a)2
= (x + y + 4a)(x + y - 4a)