+532 there is formula for this, it is 9*(9+16) is the tangent line length squared, so 225=9x^2 so x=5.
HOPE THIS HELPED!
+4625 Since this has a tangent and secant, Power Of Point gives us 9x2=225,x2=25,x=5.
+118702 Tetre got it from (3x)^2 = 9x^2 Thanks Tertre and asdf 
There is an obscure circle geometry law that says if A is a point external to the circle
then the square of the tangential distance from A to the circle
is equal to the square of the distance from A to the furthest point on any chord that is collinear with A.
Maybe that can be worded better. :/
anyway it says (3x)2=(9+16)2
No one on this post has attempted to prove this. They have simply used it to answer your question.
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