input: Write a function to add two numbers and then double the result. The function should take two parameters and return the final result. 
  
  assert double_add(2, 3) == 10
  assert double_add(0, 0) == 0
  assert double_add(-1, 1) == 0
  assert double_add(-3, -2) == -10
output: def double_add(a, b):
      return (a + b) * 2