The easiest way is to use modifiers. These features of Claris CAD allow you to pick up an object by a specific point, and snap it to a specific point on another object.
For example, if you had objects like this:
To line the arc up with the rest of the object, one easy way with modifiers is to use the endpoint modifier. Double click it to use it repeatedly, then pick the arc up by a point near, but not on the endpoint closer to the line.
If you held the arc in one position, you would notice that the arrow would not be pointing at your selection point, but would be pointing exactly at the endpoint at the upper part of the arc. The endpoint modifier would still be selected, because you double clicked, so to snap arc into place, you would release the mouse on the line.
You can use a similar procedure to resize a line to an endpoint. You would need to change the angle and length of the line so that it snaps to the endpoint of the arc. To do so you would, pick up the line by the endpoint nearest to the arc. You would see a gray shimmering line as you moved the mouse across the screen.
Now, move your mouse up the the endpoint modifier, and release the mouse on it. Now, click on the arc. You should see the line's angle and length change so that the line and the arc meet precisely. Instead of selecting the modifier by releasing the mouse on it, you may also type a keyboard equivalency for a modifier.
For the endpoint modifier, the equivalency is e. Equivalences for other modifiers are listed in the Claris CAD Quick Reference.