49(a+3b)^2-c^2
=[7(a+3b)]^2-c^2
=(7a+21b)^2-c^2
=(7a+21b+c)(7a+21b-c)
(m+n-3)^2-(3m-2n)^2
=(m+n-3+3m-2n)(m+n-3-3m+2n)
=(4m-n-3)(3n-2m-3)