a, a3 - a2 x - ax + xy
= \(a^2\left(a-x\right)-x\left(a-y\right)\)
b, 7a2-7ax-9a+9x
= \(7a\left(a-x\right)-9\left(a-x\right)\)
= \(\left(7a-9\right)\left(a-x\right)\)
c,xa-xb+3a-3b
= \(x\left(a-b\right)+3\left(a-b\right)\)
= \(\left(x+3\right)\left(a-b\right)\)