I'm developing a little game, in which I was planning to use circle-shaped enemies to shoot at. Instead of using the standard hittest, I wanted to calculate if the line (the players shot) would intersect with a circle (enemy).

After finding a method to achieve this, I tried to implement it in my code. This is giving me a headache, as it almost always gives a hit

Method

Implementation:

Enemy.prototype.globalHit = function() {
this.HitPos.x = this.Graph.Holder.Circle._x
this.HitPos.y = this.Graph.Holder.Circle._y
this.Graph.Holder.localToGlobal(this.HitPos);
};


Enemy.prototype.HitTest = function(line) {
this.globalHit();
//
var m:Number = line[0];
var b:Number = line[1];
var k:Number = this.HitPos.x;
var h:Number = this.HitPos.y;
var r:Number = 21
//
var A:Number = 1-2*k+(m*m)
var B:Number = 2*b*m-2*h*m
var C:Number = k+(b*b)-2*h*b+(h*h)-(r*r)
//
var D:Number = (B*B)-4*A*C
//
if (D>0){
return (true);

} else {
return (false);
}
};

I've checked all the stating valeus (m,b,k,h and r) and they're all correct, so there mus be a mistake in the equations. Can anybody help?