The first step is to find the reaction. In this case Hcl is a direct source of h+ ions and Naoh, OH- ions. When combined theyneutralize to form h2o and salt NaCl, which in case balances. But otherwise we would have to assign coefficients to get exactly the right number or hydrogen, chloride, sodium and oxygen moleculs before doing anything.
then find the hydroxide moles oh-. In this case it is .025(.8) where M means mole per liter. We want the equivalent of moles hcl so divide .025*.8 by 4.5 mols per liter to get liters HCl.
Hope that helps,