Answer:
Note that ABC is a 6 - 8 -10 right triangle with AB = 10
Also note that triangle ADC is similar to triangle ACB
Therefore
AD/ AC = AC / AB so
AD / 6 = 6/ 10 cross-multiply
10*AD = 6 * 6
810*AD = 36
AD = 36 / 10 = 3.6 = x
I hope this helps a little bit.