The length of the rectangle is 6 cm. What is its area if the perimeter is 18 cm?

Think like this: if one side of the rectangle is 6 centimeters long, then the second parallel side also has 6 centimeters.
6 + 6 = 12 cm – two sides of the length of the rectangle.
If the perimeter (the sum of all four sides of the figure) is 18 cm, therefore:
18 – 12 = 6 cm – two sides of the width of the rectangle.
Let’s subtract one side if we know that they are the same:
6/2 = 3 cm – the width of one side.
Knowing the length and width, we determine the area:
S = 3 * 6 = 18 cm2.
Bottom line: the area is 18 cm2.