１週間努力すれば友達ができる、とおもっていることに である
 

Please explain the particle に in １週間努力すれば友達ができる、とおもっていることに である

This Japanese sentence does not make sense as it is.

Adding "私がおどろいたのは:WATASHI GA ODOROITA NOWA;what I was surprised at was", it make sense as "I was surprised at the fact that he believes he could make friends by one week effort".

In this case, "に:NI” corresponds with "at".

Does this help?

Thank you. Here is the original image:
https://imgur.com/a/iIBr39A

Just before the sentence in question, there is "その話に河合さんは驚いた。".

The sentence in question is placed after accepting the sentence before and explaining "その話:SONO HANASHI;the story" using "に：NI" as the guide.