belt tightener thing????

OK, I tried putting the AC and powersteering back on my car myself. (I bought it without it but he gave me the parts) I think I have it right when I’m driving on the highway and clunk clunk theres goes my AC and powersteering. I decided to take it to a mechanic this tie but they said that I am missing the part that tightens the belt. I was wondering what is this part called so I can look for one.
TIA,
Tad

try AC belt tensioner.

I knew it was something like that thanks a lot.