Hey Hacker 804
the question you asked refers to the well known problem of the polar line of point P(4,5) to the conic ( in your case the given circle)
See here: Pole and polar - Wikipedia, the free encyclopedia
I forgot to give you the general form of the quation of the polar line.
If P(a,b) is any point outside of the circle x^2+y^2+Ax+By+C=0 then the equation of the polar line is ax+by+A(x+a)/2+B(y+b)/2 +C=0 .
In your case a=4,b=5 A=-4 B= 6 and C=21/2 .Apply this to find directly he equation that John stated in his post....