10/2 romex to 12/2

I have a routlet outside that was installed to charge the previous owners inlaws RV. Its powered by 10/2. I want to abandon that outlet cause there is no use for it anymore. I want it to power part of my growroom. I can splice the 10/2 with 12/2 right?

If no one here has answers someone at home depot or a local hardware store would most probably know. I do know from experience that you have to be carefull with the right guage of wire / voltage / resistance ect. I had rewired the trolling motor on my boat with the wrong guage and it damn near melted the sheathing. would have caused a fire i'm sure if not caught.