In this case, the vertex is the lowest point of the V shaped curve.
The goal is to get the stuff inside the absolute value (the 2x-5) to be as small as possible, so we can get to the lowest point. In terms of absolute value, we want the 2x-5 to be equal to 0
This allows us to set up the equation below and solve for x.
If you divide the number of serves in by 2 ( number that he lands in for every 6 serves) and then multiply by 4 ( number that land out for every 6 serves), you get 52 serves out.