Explanation:
geometric mean theorem :
height = sqrt(p×q)
with p, q being the parts of the side left and right of where the height "hits".
our height here is therefore
height = sqrt(16×65) = sqrt(1040)
and now Pythagoras for x :
x² = 65² + sqrt(1040)² = 4225 + 1040 = 5265
x = sqrt(5265) = sqrt(81×65) = 9×sqrt(65) =
= 72.56031973...