I made two-sphere geometries I will call them observer and target. I am trying to draw a line from observer to target and extend the line by 700 units. For simplicity, I am only working on the xz plane.

The observer and target move randomly.

I wrote the following code

```
var lineGeom = new THREE.BufferGeometry();
var positions = new Float32Array( 3 * 3 ); // 3 vertices per point
lineGeom.addAttribute( 'position', new THREE.BufferAttribute( positions, 3 ) );
lineGeom.setDrawRange( 0, 3 );
var material = new THREE.LineBasicMaterial( { color: 0xffffff ,linewidth:4} );
line = new THREE.Line( lineGeom, material );
scene.add(line);
tracker(marsMesh,earthMesh);
```

Tracker function: I calculate the slope of the line based on the observer and target locations then I calculate the third point at a distance of 700 units from the target.

```
function tracker(observer,target){
var positions = line.geometry.attributes.position.array;
console.log(positions)
positions = [];
positions.push(observer.position.toArray());
positions.push(target.position.toArray());
var m = (target.position.z-observer.position.z)/(target.position.x-observer.position.x);
var angle = Math.atan(m);
var tracker = new THREE.Vector3( -700*Math.cos(angle)+target.position.x, 0, -700*Math.cos(angle)+target.position.z ).toArray();
positions.push(tracker);
}
```

animate function:

```
function animate() {
var timer = 0.0001 * Date.now();
observer.position.x = Math.sin(timer * 3) * 120;
observer.position.z = Math.cos(timer * 3) * 120;
observer.rotation.y += 0.1
target.position.x = Math.sin(timer * 2) * 200;
target.position.z = Math.cos(timer * 2) * 200;
target.rotation.y += 0.1
line.geometry.setDrawRange( 0, 3 );
tracker(observer,target);
line.geometry.verticesNeedUpdate = true;
requestAnimationFrame(animate);
controls.update();
render();
}
```

I can see the observer and target spheres but I don’t see the line. What am I doing wrong? I am new to using three js.