Given S( -1 , -4 ), T( 1 , -1 ), qnd U( 6 , -5 ), reflect STU over the line y = x.

I need to know the coordinates of S', T', and U', but I'm not exactly sure what "y = x" means other than horizontal line (but I can't think of any logical answers). My only guesses so far are S'( -4 , 1 ), T'( -1 , -1), and U'( 6 , -6 ).

Math is my worst subject and any help I can get would be highly appreciated. If someone could please answer this with a detailed explanation of "this is the answer and why..." that would be great. Please no "here's the answer because [insert minimum-effort answer here]". I do need the answer, but I need the help to understand the problem even more. Thanks and have a good day!