First you look at the last two terms (-a-4)(-a+4). Since this a multiplication you multiply each components in the bracket with the other components of the bracket.
-a x -a -ax4 -4x-a -4x4
= a^2 -4a +4a -16
= a^2 -16
Now multiply by -2 and you get -2a^2+32
Hope that helps
