(a-2)^2-(2a-4)
=(a^2-4a+4)-2a+4
=a^2-4a+4-2a+4
=a^2-6a+8
=a^2-6a+9-1
=(a-3)^2-1
=(a-3+1)(a-3-1)
=(a-2)(a-4)