Help me pleasse

Which statement about the graph of y = 8(0.25)^2

Dakota earned $7.50 in interest in account a and $18.75 in interest in account b after 15 months. if the simple interest rate is 4% for account a and 5% for account b, which account has the greater principal? explain. to make it a little easier, there is an image. good luck!

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