smart_sub.rb

def smart_subtract(arr_1, arr_2, action)
  hash_1 = Hash[arr_1]
  hash_2 = Hash[arr_2]

  result = []
  hash_1.each do |key, value|
    if hash_2[key]
      result << [key, value.send(action, hash_2[key])]
    else
      result << [key, value]
    end
  end

  result.to_a
end

arr_1 = [[33, 2], [83, 1], [21, 1]]
arr_2 = [[83, 1], [33,1]]

p smart_subtract(arr_1, arr_2, "-")
# [[33, 1], [83, 0], [21, 1]]