I guess a lot of formatting is missing on your question, so these are the correct steps: using the inverse Pythagorean theorem, we know that the length of the missing side is the square root of the difference of the squares of the hypotenuse and the other leg.
Let x be the length you're looking for, we have
So, it looks like Mike's missing the last step: what he found is the square of the missing leg's length, not its actual length.