4 because 6(4) = 24 and 24-3=21
X=15
expand the brackets and collect like terms so 6x+15=7x
then -6x to get x=15
For this case we have a function of the form:

Where,
A: initial quantity of copies sold
b: decrease rate
t: time in days
Substituting values we have:

Answer:
A function that represents the book sales days after the release is:

Here u are, This is my answer