The rectangular prism shown has a length of 12 cm, a width of 12 cm, and a height of 8 cm. find the length of the inner diagonal (ad) of the prism. note that (ad) is the hypotenuse of right triangle acd. (round your answer to the nearest hundredth.)

This is simply where you apply the Pythagorean equation twice. 

Find the base hypotenuse by Pythagorean equation. 
h² = 12² + 12² = 288 

Then find the inner diagonal using that and the height. 
D² = h² + 8² 
D² = 288 + 64 
D = √352 
simplify to 4√22 
appx 18.76cm