input: Write a python function to solve a system of linear equations given matrix `a` and vector `b` and return the solution vector `x`.
  assert np.allclose(solve_linear_system(np.array([[3, 1, -9], [1, 10, 5], [2, 5, -1]]), np.array([15, 55, 0])), np.array([2, 4, 8]))
  assert np.allclose(solve_linear_system(np.array([[2, 1], [5, 7]]), np.array([11, 13])), np.array([7, -3]))
  assert np.allclose(solve_linear_system(np.array([[4, 2], [3, 1]]), np.array([20, 13])), np.array([4, 6]))
output: import numpy as np
  
  def solve_linear_system(a, b):
      return np.linalg.solve(a, b)