Please help! adding/multiplying negative numbers

Find two rational expressions that have the difference of 2-n/n-4

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

Look at the figure below: triangle abc is a right triangle with angle abc equal to 90 degrees. the length of ac is 5 units and the length of ab is 4 units. d is a point above c. triangle adc is a right triangle with angle dac equal to 90 degrees and dc parallel to ab. what is the length, in units, of segment cd?