code: def x_axis_rotation(theta):
          
          R = np.array([[1, 0, 0,],
                        [0, np.cos(theta), -np.sin(theta)],
                        [0, np.sin(theta), np.cos(theta)]])
          return R
docs: Generates a 3x3 rotation matrix for a rotation of angle
          theta about the x axis.
  
          Parameters
          ----------
          theta : float
              amount to rotate, in radians
  
          Returns
          -------
          :obj:`numpy.ndarray` of float
              A random 3x3 rotation matrix.