49(a-b)^2-(c-d)^2
This is the difference of two perfect squares, which can also be written as the product of the sum and the difference of the two square roots, which in this case is
[7(a-b) + (c-d)][7(a-b) -(c-d)]
= (7a -7b +c -d)(7a -7b -c + d)
[7(a-b) + (c-d)][7(a-b) -(c-d)]
= (7a -7b +c -d)(7a -7b -c + d)