input: Write a python function to calculate and plot a linear regression line using given x and y arrays.
  assert round(b1, 2) == 1.16
  assert round(b0, 2) == 1.74
output: import numpy as np
  import matplotlib.pyplot as plt
  
  def linear_regression(x, y):
      n = len(x)
      mean_x = np.mean(x)
      mean_y = np.mean(y)
      SS_xy = np.sum(y*x) - n*mean_y*mean_x
      SS_xx = np.sum(x*x) - n*mean_x*mean_x
      b1 = SS_xy / SS_xx
      b0 = mean_y - b1*mean_x
      
      plt.scatter(x, y, color="m", marker="o", s = 30)
      y_pred = b0 + b1*x
      plt.plot(x, y_pred, color="g")
      plt.xlabel('x')
      plt.ylabel('y')
      plt.show()
      
      return b0, b1