Finding your partner Challenge

Given an arbitrary list of 2-tuples, and a single element in one of those tuples, output its "partner", i.e. given a and [(i,j),...,(a,b),...,(l,m)], output b. You can assume all tuples are unique, and that all elements in tuples are strings. Further assume you do not have both (x,y) and (y,x).

Test cases

Input                                                           Output

[("(", ")"), ("{", "}"), ("[", "]")], "}"                       "{"
[("I'm", "So"), ("Meta", "Even"), ("This", "Acronym")], "Even"  "Meta"
[("I", "S"), ("M", "E"), ("T", "A")], "A"                       "T"
[("test", "cases"), ("are", "fun")], "test"                     "cases"
[("sad", "beep"), ("boop", "boop")], "boop"                     "boop"


Fewest bytes wins!
Japt, 6 bytes
Works with strings or integers.
æøV kV

Test it

Explanation
Implicit input of array U and string/integer V.
æ

Get the first element (subarray) in U that ...
øV

Contains V.
kV

Remove V and implicitly return the resulting single-element array.