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