x²-3ax-6ab-4b²
=x²-3ax+9/4a²-9/4a²-6ab-4b²
=x²-3ax+9/4a²-(9/4a²+6ab+4b²)
=(x-3/2a)²-(3/2a+2b)²
=(x-3/2a+3/2a+2b)(x-3/2a-3/2a-2b)
=(x+2b)(x-3a-2b)