Answer:
Part 1) The expression is

Part 2) The area of the schoolyard when x=40 m is A=2,800 m^2
Part 3) The domain is all real numbers greater than zero and less than 75 meters
Explanation:
Part 1) Write an expression for A(x)
Let
x -----> the length of the rectangular schoolyard
y ---> the width of the rectangular schoolyard
we know that
The perimeter of the fencing (using the wall of the school for one side) is


so
-----> equation A
The area of the rectangular schoolyard is
----> equation B
substitute equation A in equation B


Convert to function notation

Part 2) What is the area of the schoolyard when x=40?
For x=40 m
substitute in the expression of Part 1) and solve for A

Part 3) What is a reasonable domain for A(x) in this context
we know that
A represent the area of the rectangular schoolyard
x represent the length of of the rectangular schoolyard
we have

This is a vertical parabola open downward
The vertex is a maximum
The x-coordinate of the vertex represent the length for the maximum area
The y-coordinate of the vertex represent the maximum area
The vertex is the point (37.5, 2812.5)
see the attached figure
therefore
The maximum area is 2,812.5 m^2
The x-intercepts are x=0 m and x=75 m
The domain for A is the interval -----> (0, 75)
All real numbers greater than zero and less than 75 meters