N = 20 + 7y
Joe is starting with 20 Apple trees, so the equation starts with:
N = 20
And each year afterwards, 7 more trees will be planted, so let's represent that as 7y where 7 is the number of trees to plant each year and y is the number of years that this has been going on. Finally, add that to the initial tree count, giving:
N = 20 + 7y
