< 7 ==> 7 - 5 ==> 2
Pythagorean Theorem: C^2 = A^2 + B^2, or A^2 + B^2 = C^2
< AD <^2 ==> 2^2 + 18^2
< AD <^2 ==> 4 + 324
< AD <^2 ==> 328
< AD ====> 18.1
Hence, The Answer: Letter Choice, (A), 18.1
Hope that helps!!!! : )
