input: Write a python function to calculate the coefficients of a linear regression model.
  assert calculate_coefficients([1, 2, 3], [2, 2.5, 3.5]) == (1.5, 0.5)
  assert calculate_coefficients([1, 2, 3], [3, 4, 5]) == (2.0, 1.0)
  assert calculate_coefficients([1, 2, 3, 4], [1, 2, 3, 4]) == (0.0, 1.0)
output: def calculate_coefficients(x, y):
      n = len(x)
      sum_x = sum(x)
      sum_y = sum(y)
      sum_xy = sum(xi * yi for xi, yi in zip(x, y))
      sum_x_squared = sum(xi**2 for xi in x)
      
      b = (n * sum_xy - sum_x * sum_y) / (n * sum_x_squared - sum_x**2)
      a = (sum_y - b * sum_x) / n
      
      return a, b