How do I get the keys of a hash whose values equal to given arguments in ruby?

How do I get the keys of a hash whose values equal to given arguments in
ruby?

So I am trying to solve this problem from rubeque
http://www.rubeque.com/problems/related-keys-of-hash/. Basicaly I just
have to get keys of a hash whose values equal to given arguments.I was
wondering if you guys can give me some hints? to solving this problem,
thank you very much
this is what I have so far
class Hash
def keys_of(*args)
key = Array.new
args.each { |x| key << x}
key.each { |x,y| x if y == key}
end
end
assert_equal [:a], {a: 1, b: 2, c: 3}.keys_of(1)
assert_equal [:a, :d], {a: 1, b: 2, c: 3, d: 1}.keys_of(1)
assert_equal [:a, :b, :d], {a: 1, b: 2, c: 3, d: 1}.keys_of(1, 2)