+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!
+4609 Since this has a tangent and secant, Power Of Point gives us \(9x^2=225, x^2=25, \boxed{x=5}.\)
+118609 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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