To find the average rate of change in a interval
[tex]a\leq x\leq b[/tex]We use the following formula:
[tex]\frac{f(b)-f(a)}{b-a}[/tex]Applying this to our interval
[tex]6\leq x\leq15[/tex]We have the following rate of change:
[tex]\frac{f(15)-f(6)}{15-6}=\frac{107-53}{9}=\frac{54}{9}=6[/tex]We have an average rate of change equals to 6.