An illusion is a type of

The graph below shows the amount of money left in the school’s desk fund, f, after d desks have been purchased. for each new desk that is purchased, by how much does the amount of money left in the school’s desk fund decrease?

Find the constant of variation for the relation and use it to write an equation for the statement. then solve the equation.

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