Ive been trying to solve this for hours.
That's a sad state of affairs. However, if true, you probably have lots of work on paper. Why not show us some of it.
I don't know how to write the equation,
That is also unfortunate. Generally, an instructor gives a problem after teaching the basics. Maybe you could do with a review of your textbook and class notes.
or how to use the water.
We'll work on this as we go.
An unknown amount of oxalic acid is dissolved in 50.0 mL of water. If it takes 21.77 mL of a 0.1500 M solution of lithium hydroxide to neutralize the oxalic acid solution, what is the molarity of the oxalic acid solution?
Have you done a similar problem? Or can you find one in the text book?
Write the balanced chemical equation.
Always a good first step.
{MOD Edit: new title}