Answer :

This is simply a difference of squares. To factor this, you simply take the square roots of the leading coefficient and the C term. Thus you get 2 and 5. As a pair of factors, it is simply (2a+5)(2a-5). If you actually need to solve this, you can set either the factors or the original equation equal to 0.
magic
4a² - 25

= (2a)² - 5²

= (2a+5)(2a-5)        since a² -b² = (a+b)(a-b)