The perimeter of the rectangle is 84 cm, the width of the rectangle is 4 cm, which is equal to the length of the rectangle.

We know that the perimeter of the rectangle is 84 cm, and the width of the rectangle is 4 cm. We need to find what is the length of the rectangle.

Let’s remember what perimeter is in mathematics. The perimeter of a geometric figure is the sum of the lengths of all sides of a given geometric figure.

For a rectangle, the following formula is used:

P = 2 (a + b);

where a and b are the length and width of the rectangle, respectively. Substitute the known values and get the equation:

2 (a + 4) = 84;

2a + 8 = 84;

2a = 84 – 8;

2a = 76;

a = 38 cm rectangle length.