Verified Answer :

Somewhere around 45-46 inches
Hope this helps

so 2/3 of 5 feet and 9 inches
lets convert that to only inches
so 69 inches

now 2/3 OF 69
(btw in math of means )
so 
2/3
69 
that is 46
the dog is 46 inches tall
or converted back into feet and inches
3 feet and 10 inches

That's a tall dog lol

Hope this helps :)