The domain of a function is defined as the set of values of x that will give out a real value of y, a "real" number is basically a number that can be expressed as itself instead of as a symbol or a calculation, for example -4 would be a real number as we can simply express it as -4, however √-4 would not be a real number as it would be expressed as 2i. For the function y=25-x all values of x will result in a real number therefore we say domain of f(x) is x∈R, or simply saying "the domain of the function y=25-x is all real values of x" will probably suffice.
