A rectangle whose length is 5.3 cm and whose width is 2.5 cm.

would my answer be 7.8 centimeter?

it would if you want half the perimeter.

SO what do I need to do?

To find the area of a rectangle, you need to multiply its length by its width. In this case, the length is 5.3 cm and the width is 2.5 cm.

Using the formula:
Area = Length x Width

You can substitute the values into the formula:
Area = 5.3 cm x 2.5 cm

To get the result, simply multiply the numbers:
Area = 13.25 cm²

Therefore, the area of the rectangle with a length of 5.3 cm and a width of 2.5 cm is 13.25 square centimeters.