Factorise by grouping method:
a(a−4)−a+4
a(a−4)−a+4 =a(a-4)-1(a-4) =(a-4)(a-1)
16(a+b)2−4a−4b
a4−2a3−4a+8
a2+4b2−3a+6b−4ab
x(6x−5y)−4(6x−5y)2
y2−(a+b)y+ab