GRE Big Book T10 S7 Q15

Hey guys, need some help. I can choose numbers and get the length of the diagonal SU, what about the height TV that I’m not getting and what foundational concep is escaping me ? Grateful for your explanations

Hey guys, need some help. I can choose numbers and get the length of the diagonal SU, what about the height TV that I’m not getting and what foundational concep is escaping me ? Grateful for your explanations