Answer: She added 7x and 3y
Step-by-step explanation: The final answer should be 7x + 3y, not 10xy.
Here it is given that the width is x ft and total length of the fence is 2400 ft .
Let the length be y ft
So we have

Let A represents area, and area is the product of length and width .
So we get

Substituting the value of y, we will get

Second part
The area is maximum at the vertex, and vertex is

And

And that's the required dimensions .
Answer:
6
Step-by-step explanation: