Question 752751
{{{ (a-(2b-c))(a+(2b-c))}}}  step 1


{{{ (a-2b+c)(a+2b-c) }}}     step 2


{{{ a*a+2b*a-c*a-2b*a-2b*2b-2b*(-c)+c*a+c*2b-c*c }}}     step 3...multiply


{{{ a^2+2ab-ac-2ab-4b^2+2bc+ac+2bc-c^2 }}}


{{{ a^2+cross(2ab)-cross(ac)-cross(2ab)-4b^2+2bc+cross(ac)+2bc-c^2 }}}


{{{ a^2-4b^2+4bc-c^2 }}}.....you can have this as an answer, or  


{{{ a^2-4b(b-c)-c^2 }}}