Multiplying a fraction by a whole number 4/12,4/12,4/12,4/12,4/12

Monica build a remote control toy airplane for science project to test the plane she launched it from the top of a building the plane travel a horizontal distance of 50 feet before landing on the ground a quadratic function which models the height of the plane in feet relative to the ground at a horizontal distance of x feet from the building is shown

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

Matt had 5 library books. he checked out 1 additional book every week without retuening any books. whats the matching equation