If x/y = 2/3, then all we have to do is multiply x/y by 6. Another way to look at this (the same thing) would be to see 6 as 6/1, so 6 is multiplied by x, and y is multiplied by 1.
But, all the same, you multiply 6 by x/y and you get (2·6)/3 = 12/3 = 4
