Hmm... Well that would be a very involved answer. You should be able to handle it if you know basic algebra. I'm assuming you were given the values to plug in. Also it assumes that it's pure water. Impurities such as salt require the chemical process to first break the hydrogen bonds making water boil at a higher temperature.
I did it for you quickly and got an an answer of 18,900m rounded.
Edit: Actually that can't be correct. That's the limit where water boils at the internal temperature of a human, 37 C. I must have messed up my arithmetic.