Guess how it goes if you are talking volume and the linear ratio is 3:5!
The three and the 5 were the only numbers we were given- that's why we "square only the 3 and 5"!
The area of any plane figure involves multiplying together two lengths- that's why the ratio between lengths is squared.
The volume of any solid figure involves multiplying together three lengths so to find the ratio of volumes when we are given a ratio of lengths, we cube.