You use the Pitagora formula:
CD^2+BD^2= (6x-10)^2
CD^2+AD^2=16x^2
AD^2-BD^2=16x^2-(36x^2-120x+100)
0=-20x^2+120x-100
x^2-6x+5=0
x^2-x-5x+5=0
x(x-1)-5(x-1)=0
(x-1)(x-5)=0
x1=1 but if x=1 it means that BC=-4 which cannot be possible
x2=5 so x=5
The answer to your question is C. I hope that this is the answer that you were looking for and it has helped you.
