input: Write a function to calculate the distance between two points on a two-dimensional plane. The function should take four parameters: x1, y1 for the coordinates of the first point, and x2, y2 for the coordinates of the second point. The function should return the distance as a float.
  
  assert calculate_distance(3, 4, -9, -2)==13.0
  assert calculate_distance(0, 0, 1, 1)==1.4142135623730951
output: def calculate_distance(x1, y1, x2, y2):
      return ((x2 - x1) ** 2 + (y2 - y1) ** 2) ** 0.5