What is the area of a rectangle that measures 9cm by 81mm?

Well, a Centimeter is 10 times bigger than a Milimeter.

So we can convert 9cm by multiplying it by 10

9 * 10 = 90mm

SInce we know the length and width of the rectangle, we can now find the area of it, or "the space inside".

Area = length * width

Area = 90mm * 81mm

Area = 7290mm