How to Get Both Coordinates In a Nested Array?

I'm working on a chess game and want to search the nested array for the King piece K(b)and then return the coordinates.

I've tried the following method which returns 4 (the x coordinate - the index in the internal array). However, I'm trying to figure out how to also return index of the outer array. Expected output is [4, 7]:

piece = "K(b)"

def coordinates_for_piece(piece)
  @board.map { |item| item.find_index(piece) }
end

Board:

def piece_setup
    @board = Array.new(8) { Array.new([" .  ", " .  ", " .  ", " .  ", " .  ", " .  ", " .  ", " .  "]) }

    @board[0] = ["R(w)", "N(w)", "B(w)", "Q(w)", "K(w)", "B(w)", "N(w)", "R(w)"]
    @board[1] = ["P(w)", "P(w)", "P(w)", "P(w)", "P(w)", "P(w)", "P(w)", "P(w)"]
   
    @board[6] = ["P(b)", "P(b)", "P(b)", "P(b)", "P(b)", "P(b)", "P(b)", "P(b)"]
    @board[7] = ["R(b)", "N(b)", "B(b)", "Q(b)", "K(b)", "B(b)", "N(b)", "R(b)"]
end

EDIT: This is a VERY clunky way of accomplishing the goal. Looking for something more streamlined.

def coordinates_for_piece(piece)
   test = @board.map { |item| item.find_index(piece) }
   => [nil, nil, nil, nil, nil, nil, nil, 4]
   x = test.find{|x| !x.nil?}
   => 4
   y = test.find_index{|y| !y.nil?}
   => 7
   [x, y]
end