The domain is just what values of x the function is defined for. So if there is a value of x that gives an "undefined" value of why then its not part of the domain.
Just think about what values of x would make the parts under the square root a negative (which isn't defined/possible).
So if your teacher said x>3, test a value (say 4) and see what happens. it would be sqrt(2) - sqrt(-1) which isn't possible.
But a value less than 2 would make the first part a square root of a negative number.
So we need a number greater than 2 and less than 3.
Hence the answer you've been told.