Please Help (math)

We can use  the  tangent-secant theorem here

This says that  the square  of the tangent segment  =   the length of the external part of the secant segment * the  length of the secant 

We  have  that

8^2  =  4 ( 4 +x)  simplify

64  = 16 + 4x      subtract  16 from both sides

48 = 4x     divide both sides by 4

12  =  x