Actually Welcome to the concept of Geometric Mean (leg) theorem.
So according to the rule, we know that, here altitude = h and left side is 6 and right side is 14 , so we get as,
==> altitude/left = right/altitude
==> h/6 = 14/h
==> h^2 = 14*6
==> h^2 = 84
2.) so now we apply the Pythagoras theorem in triangle ∆DCB
so we get as,
==> (14)^2 + h^2 = a^2
==> 196 + 84 = a^2
==> 280 = a^2
==> a = √280
==> after simplified => a = √70*4
==> a = 2√70 units
hence the correct answer is a => 2√70 units.