Explanation:
I just answered this. how often did you post that ?
the inner height and the base area radius create a right-angled triangle with l.
so, Pythagoras can be used :
l² = 7² + (14/2)² = 2×49 = 98
l = sqrt(98) ft
FYI : this could be further simplified :
sqrt(98) = sqrt(2×49) = 7×sqrt(2)