Rocky needs to save an additional $15 to buy the amplifying speaker, starting with the $17 he already has. The graph representing his savings should start at $15 and continue upwards without bound.
The student's question revolves around the amount of money Rocky needs to save to buy an amplifying speaker. Rocky already has $17 but requires at least $32 to make the purchase.
Therefore, he must save an additional $15 to meet the minimum requirement.
Since there is no upper limit on how much more he can save over $32, the graph representing the possible savings would start at $15 and extend indefinitely to higher amounts.
The $15 is arrived at by subtracting the amount Rocky already has from the total needed: $32 - $17 = $15.