1-25x^2

This is a difference of squares, in the form of $$a^2 - b^2 = (a-b)(a+b)$$.

Here, $$a^2 = 1$$, so $$a = 1$$. Also, $$b^2 = 25x^2$$, so $$b=5x$$.

Thus, the factored form is $$(1-5x)(1+5x)$$.