Inverse of f(x)=-1/3x+7 (with steps included)

Diana is painting statues. she has 7/8 of a liter of paint remaining. each statue requires 1/20 of a liter of paint.how many statues can she paint? yeah dad i added the cup to the door now will you stop emailing me while im doing my homework.(lmbo xd i just realized this wooow)

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

Witch represents the inverse of the function f(x)=1/4x-12