How do you align a part between two points using a weld or CFrame?

Hello everyone, I have been developing a gun engine, but am having problems writing the code that poses the character's arms. What I want to do is to align the right arm between the trigger and the player's right shoulder, and the left arm between the underbarrel of the gun and the player's left shoulder. I am not asking for a complete solution, but maybe a few pointers to get me started. (I am in highschool and have completed algebra one , just indicates my math level)

Any help is greatly appreciated! --Cantos