To find the side b you would use the pythagorean theorem, which is a squared + b squared = c squared. That means it would be 7 squared + b squared = 9 squared. 49+b squared=81. subtract 49 on both sides and u would get b squared=32. Square root of 32 is about 5.6569.
Answer is 5.6569 or 5.66.
Hope this helps.