see the picture attached to better understand the problem
we know that
If two secant segments are drawn to a circle from an exterior point, then the product of the measures of one secant segment and its external secant segment is equal to the product of the measures of the other secant segment and its external secant segment.
so
jl*jk=jn*jm------> jn=jl*jk/jm
we have
jk=8,lk=4 and jm=6
jl=8+4----> 12
jn=jl*jk/jm-----> jn=12*8/6----> jn=16
the answer is
jn=16
