Here is a graph that will explain
The x intercept of this graph is 15, the y intercept is 12
This means that when you have 0 roses you will have 15 tulips and when you have 0 tulips you will have 12 roses. Any point that falls on this line can be a correct combination of roses and tulips. You probably only want to look at whole number answers because you don't want to buy .5 roses or tulips. Hope this helps.