x=3.95
Explanation
From the given information we find that the triangle is right angled at C. CB is the base,AC is the height and AB is the hypotenuse.
Given that D is the hypotenuse of the hypotenuse AB.
BC=6
BD=4.2
AD=x
the height and the base of the triangle is given.
Using pythagoras theorem, we can find the hypotenuse AB.

D is the midpoint of AB
therefore AD=AB/2=
=3.905
x=3.905 cm
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