Well since we're dealing with real numbers, we aren't allowed to take the square-root of a negative number. So find out for what x-values the expression inside the square-root is less than 0.
Well, it's not that hard to find the "why."
Let's use our brain: having For what values of is well defined? Yes, we require that 'cause can be zero, or positive, but it can't be negative, since square roots of negative numbers don't belong to the Real numbers, that's why we require
Now, by applyin' this to your problems, you'll see this does make sense.