The shape of the field is rectangle
The perimeter of a rectangle is the total length of all four borders.
x + x + y + y = 1040/3 ⇒ Collecting like term
2x + 2y = 1040/3 ⇒ We want to isolate 'y', so move '2x' to the right hand side of equation
2y = 1040/3 ÷ 2x
y =

÷ 2
y = 1040/12x