You want to find out where the function is not defined. You are taking the square root of w. So w must be greater than or = to 0. Next, you cannot divide by zero. Therefore the square root of w cannot be 5. So what is 5 squared? w cannot be that number. Therefore you have a domain something like: [0, 25) U (25, ∞). What does that mean? you have a split domain. Since 0 is included, we put a bracket on 0. 25 is not included, so you put a parenthesis around 25, and then define the rest of the domain, with a U for union, since once you get above 25 there are no limitations.