a)x3-10x-12
=x3-4x-6x-12
=x.(x2-4)-6.(x+2)
=x.(x-2)(x+2)-6.(x+2)
=(x+2)[x.(x-2)-6]
=(x+2)(x2-2x-6)
b)x3-7x-6
=x3-x-6x-6
=x.(x2-1)-6.(x+1)
=x.(x-1)(x+1)-6.(x+1)
=(x+1)[x.(x-1)-6]
=(x+1)(x2-x-6)
=(x+1)(x2+2x-3x-6)
=(x+1)[x.(x+2)-3.(x+2)]
=(x+1)(x+2)(x-3)