If I've read your question correctly, you need to subtract: a2 +2a -7
a2 -4a2 +5a2 -6 = 2a2 -6
Note, if x - y = z, then y = x - z; so:
2a2 -6 - (a2 -2a +1) = 2a2 -6 - a2 +2a -1
= a2 +2a -7
It depends. If you want to destroy the grabacr/ofnir mix squadron easiest use a plane with xlaa or xmaa(preferred x-02 or f/a22a) and the sp color ace replicator, it will pop up after you destroyed the 8492nd and easiest way to kill him, attack head on