How many milliliters of 2m sodium hydroxide are needed to neutralize 15ml of 6 hydrochloric acid?
I will assume (I know that's dangerous) that you actually mean 2 molar, which should be written 2 M. "2 m" would be 2 molal, and acid-base neutralization reactions almost always use molarity concentrations.
Molarity is defined as the number of moles of solute per liter of solution:
M = mols/L, or M = mols/Volume
(The "M" is usually underlined or italicized.)
A neutralization reaction will be complete when the number of hydrogen ions from the acid equals the number of of hydroxide ions from the base.
If we rearrange the molarity definition, mols = M x V.
mols of H+ must equal mols of OH-, so we can set up an equation
M(acid) x V (acid) = M(base) x V(base). You have three of these four values, so you can solve for the fourth.
If you have substances with more than one hydrogen or hydroxide ion, you will have to account for that.
M(acid) x V (acid) X H+/acid molecule = M(base) x V(base) x OH-/base molecule. (Using "molecule" rather loosely there.)
Using this equation you should also notice that is not necessary to convert from mL to L and back. The units on each side will cancel, so you can use mL, L,