The range of a function is the range of values which the function can take given it's domain.
As the domain is not given, assume that it is all values of x which give a real y (in this case
Three important facts to remember in this question are that:
a) you can only take the square root of a positive number
c) any real number squared
Using a), you know
Using a) the minimum value
Given the minimum and maximum it follows that