The quote is actually slightly fuzzy because it's only in the movie version, and I believe he's not facing towards the camera while he speaks but it's something like this:

You just begin. You do the math, you solve one problem. Then you solve the next one, and then the next and if you solve enough problems you get to come home.

Here's the quote above:

At some point everything's going to go south on you. You're going to say, "This is it. This is how I end." Now you can either accept that, or you can get to work. You solve one problem, and then you solve the next problem, and the next, and if you solve enough problems, you get to go home. ~Mark Watney, The Martian