(3a-2b-c)^2-(3a+2b-c)^2
=[(3a-2b-c)+(3a+2b-c)][(3a-2b-c)-(3a+2b-c)]
=(3a-2b-c+3a+2b-c)(3a-2b-c-3a-2b+c)
=(6a-2c)(-4b)
=-24ab+8bc