Question 600381
I think the triangle should be a right angled triangle.
If it is, then we can find the third side by applying the Pythagoras Theorem.

In that case, the third side of the triangle would be
{{{sqrt( 17^2-15^2 )}}}
i.e. {{{sqrt( 289-225 )}}}
i.e. {{{sqrt( 64 )}}}
i.e. 8cm