Answer
number of vines $n = 850$
maximum grape production $A(n)= 7225$
Work Step by Step
Calculate the maximum of the function by substituting the values from the function into $n_{MAX}=\frac{-b}{2a}\rightarrow n_{MAX}=150$ which is the number of additional vines.
Then substitute $n=150$ into the function to get the maximum grape production.
Since n represents the number of additional vines only, we should add them to the original number of vines planted $\rightarrow 700+150=850$