There are 2 short sides x and 1 long side y.
x+x+y=1,800
2x+y=1,800
y=1,800-2x
The area of the rectangle is A=x(1,800-x).
A is a function with variable x. Moreover it is a quadratic (second degree) function.
The graph of the function is a parabola which opens downwards, because the signs of the leading coefficients of the factors, multiply to minus:
(x)(-x)=-x^2.
This means that the highest point of the parabola, is its vertex V(h,k), so the function takes its largest value for x=h.
The roots of A=x(1,800-x) are x=0 and x=1,800, thus the x coordinate of the vertex, must be the midpoint of 0 and 1,800, that is 900.
Answer:
A=x(1,800-x)
largest area is when x=900
