Joe owns an apple orchard with 20 apple trees. each year he will plant 7 more. write an explicit formula for the number of apple trees in the orchard

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

Xis -1/2
y is 35/4.