Question 43995
{{{a^2+b^2=c^2}}}
{{{9^2+b^2=17}}}
{{{81+b^2=17}}}
{{{b^2=-64}}}
this is geometrically impossible, as the square root of -64 cannot exist.
I think you may have labelled the sides of the triangle incorrectly. Always call the known lengths "a" and "b". Try: 
{{{a=9}}}
{{{b=sqrt(17)}}}
this should give {{{c=sqrt(88)}}}

I hope this helps.

P.S. I am trying to start my own mathematics homework help website.  I would be extremely grateful if you would send me your feedback on my help to adam.chapman@student.manchester.ac.uk.