Answer:
6
Step-by-step explanation:
because every 7 books Miss. Nelson gets 2 free books so when Miss. Nelson bought the 7 books she got the 6 and also took 21 divided by 7 and you get 3 then you times that with the 2 and that is how you get 6 books for free
we have the function

we know that
In the equation of the exponential function of the form

a is the initial value
b is the base
x is the exponent
The initial value is the value of the function when the value of x is equal to zero
in this problem
for 


the initial value is the point 
therefore
the answer in the attached figure