You want to be finding out what two factors (where one of them is a perfect square) has the produce of 63. It seems like 9 x 7 (9 is the perfect square) works.
[tex] \sqrt{63} = \sqrt{9 \times 7} = \sqrt{9} \times \sqrt{7}[/tex]
The square root of 9 is 3 so we can just have [tex]3 \sqrt{7} [/tex] (which just means 3 times the square root of 7).
