5minus 1 whole and 1/4 equals 4 plus 5 minus 1

Let f(x) = 3 − x . find the average rate of change of f(x) from x = a to x = a + h and simplify your answer so that no single factor of h is left in the denominator.

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?

Type the correct answer in the box. use numerals instead of words. if necessary, use / for the fraction bar.