The average rate of change from an interval from [tex]x_1[/tex] to [tex]x_2[/tex] is the change in the value of the function divided by the change in x.
[tex]\dfrac{f(x_2)-f(x_1)}{x_2-x_1}[/tex]
Let's find the values of [tex]f(x_1)[/tex] and [tex]f(x_2)[/tex]
[tex]f(x_1)=4^2+3(4)=28[/tex]
[tex]f(x_2)=9^2+3(9)=108[/tex]
[tex]\dfrac{f(x_2)-f(x_1)}{x_2-x_1}[/tex]
[tex]=\dfrac{108-28}{9-4}=\boxed{16}[/tex]
Your answer is 16. I hope this helps and have an awesome day! :)