Алгебра 8. 3x(x+y+c)-3y(x-y-c)-3c(x+y-c)

3x(x+y+c) - 3y(x-y-c) - 3(x+y-c) =
3x^2 +3xy+3xc-3xy+3y^2 +3yc-3xc-3yc+3c^2 =
3x^2 +3y^2 +3c^2 =
3(x^2 +y^2 +c^2)