Look at the picture.
The triangle ADC and the triangle CDB are similar.
Therefore corresponding sides are all in the same proportion:

Answer: 4√3
Recall the secant-tangent theorem, and you have
EA^2 = EC*CD
12^2 = 8*(x+10)
and now ED = EC+CD = 8+x+10
I suspect a typo somewhere in the murk above
Answer:
16
Step-by-step explanation:
The variable a minus 10 equals 6. If you substituted a for 16, and subtracted 10, you'd get 6. Substituting the rest of the numbers and subtracting 10 wouldn't give you 6.
That's correct because 13•-1•-5=-18 so what is the question