What symbol in the above example stands for the unknown number?

Afruit stand has to decide what to charge for their produce. they need $10 for 4 apples and 4 oranges. they also need $15 for 6 apples and 6 oranges. we put this information into a system of linear equations. can we find a unique price for an apple and an orange?

Find the equation using the point (4,8)

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