Since you know two sides and are trying to find the missing side, you can use the Pythagorean Theorem. So fashion the equation like this:
x^2 + 3^2 = sqrt10^2. After simplify, which leads to x^2 + 9 =10. Then have the variable to one side by subtracting 9 on both sides. This leads to x^2=1. Finally, square root both sides which leads to 1. Note that any root of 1 is 1. So your answer becomes x=1.
