Because the three horizontal lines are parallel, so the portions of transversal lines are equal in length like PQ = QR = RS
As we are given that WY=2.25, that means WX = 2.25*2=4.5
And as the line TU is inclined more than WV, so the segment confined by two parallel lines has to be longer than 2.25....
That leads us to that the length of XU should be greater than 4.5, so we can pick 6 confidently :)
Hope you got what want to ask about