Find the side of a rectangle if the length and width together are 2100 meters, and the width is half the length.

We denote by X the width of the rectangle.

We denote by Y the length of the rectangle.

By the condition of the problem, the length of the rectangle is 2 times its width. We express the length in terms of the width:

Y = 2X.

By the condition of the problem, the area of the rectangle is 2100 m. We compose and solve the equation:

X + 2X = 2100;

3X = 2100;

X = 2100/3;

X = 700.

Find the second side of the rectangle:

Y = 2X = 2 * 700 = 1400.

Answer: the width of the rectangle is 700 m and the length is 1400 m.