Step-by-step explanation:
I think you went through right-angled triangles and Pythagoras before you stated these topics ?
then it is really easy.
as the graphic suggests already : the inner height of the cone, the radius (half of the diameter) of the base circle and the mantle length l build a right-angled triangle, and we can use Pythagoras to get l :
l² = 7² + (14/2)² = 7² + 7² = 2×49 = 98
l = sqrt(98) ft
FYI : which could be further simplified as 98 = 2×49 :
sqrt(98) = sqrt(2×49) = 7×sqrt(2)
