input: Write a function that takes two numbers as input, adds them together, and then doubles the result.
  
  assert double_add(2, 3) == 10
  assert double_add(5, -2) == 6
  assert double_add(0, 0) == 0
  assert double_add(-1, -1) == -4
  assert double_add(10, 10) == 40
output: def double_add(a, b):
      return (a + b) * 2