If the rectangles are similar, then all of their linear dimensions have the same ratio ... length, width, perimeter, diagonal, etc., For these two rectangles, anything in the bigger one that you can measure with a ruler is 3 times as long as the same measurement in the smaller one.
Their areas will have the square of that ratio. For these two rectangles, the area of the bigger one is (3²) = 9 times the area of the smaller one.
