The domain of a function is your input or x and the range is the resulting y
If small values map to small values, then with growing input the result also grows or in other words the function has a positive slope.
The assignment wants you to find a linear function which does not do that, or in other words maps small x to large y and large x to small y. With growing x the y is therefore reduced and you have a negative slope. So the real question is "name any function with a negative slope". In this case with every step the range value gets smaller and so the first value can't be the least/smallest of the range.
so f(x)=-x would be solution
if you have to specify a domain and range you can pick something like D=[1,5] and R=[1,-5]