Veronica is mulching her front yard. the total weight of mulch varies with the number of bags of mulch what is the rate of change?

I saw the graph that should have accompanied this problem.
y axis = weight in pounds ; intervals of 30.
x axis = number of bags, intervals of 1.

Since the graph forms a line, the rate of change is constant. y = kx
 x     y
1     30
2     60
3     90
4    120
5    150

Rate of change = weight(lb) / bags = 30/1  or 30 pounds per bag.