Completely factor the polynomial, if possible. \[ 25 y^{2}-x^{2} \]

Step 1 ：This is a difference of squares problem. The difference of squares formula is a^2 - b^2 = (a - b)(a + b). In this case, a = 5y and b = x.

Step 2 ：So, we can factor the polynomial as (5y - x)(5y + x).

Step 3 ：The factored form of the polynomial is \(\boxed{-(x - 5y)(x + 5y)}\).