Just make the substitutions and the squareroot of both sides.
for ||3X-4Y||
knowing that ||X||=2 AND ||Y||=1 and X.Y=-1
and then compute for
(3X+4Y).(X-5Y)
apparently the answer for the first is square root of 76
and the second is 1
I tried doing this the way the book tells me but I don't get the right answer at ALL!