The domain is the set of all possible numbers you can use for x that don't turn the equation into some sort of illegal statement. For example, if f(x)=1/x we know that x can't equal 0, because division by zero isn't allowed. So the domain would be all numbers except 0.

So can you think of any values for x that make f(x)=-7 illegal or invalid?

If it seems like the answer is so easy that it must be a trick question, it's because it really IS that easy. The right side of the equation doesn't even contain any x's at all! That means that no matter what value you put for x, whether it's 1, 2, 100, -1000000, or 44538743593974.323424, f(x) will always, always be -7 (which is a perfectly legal value). So the domain of f(x) is -infinity to infinity.

Does that make sense?