If side AC on triangle D was 8 inches long and side BA on triangle D was 17 inches long how long is side BC?

Your triangle is described well enough.  I assume it is triangle ABC but what is D  ??

Assuming that we have a right triangle with BA as the hypotenuse....

BC = √ [ BA^2 -AC^2]  = √[ 17^2 - 8^2]  = √ [289 - 64]  = √225   =  15