you can do this 2 ways, one is knowing the special right triangles called triplets. An example of a triplet would be 3-4-5 where the largest value is the hypotenuse. In this example, the other 2 sides are 3 and 4 and the missing side is the hypotenuse, so using the knowledge of triplets, we can find that the hypotenuse is 5.
algebraically, we can use the Pythagorean theorem; a^2+b^2=c^2 where a and b are the opposite and adjacent and where c is the hypotenuse. We can then substitute in the values to get : 3^2 + 4^2 = c^2. When simplified, we get 9+16=c^2. Simplify further, and get 25=c^2. Now we Square root both sides to solve for C. Finally, we get C=5. Your missing side is 5.