input: Write a python function to solve a system of linear equations and return the values of x and y.
  assert solve_linear_eq(1, 2, 3, 4) == (1.0, 0.0)
  assert solve_linear_eq(2, 1, 1, 2) == (0.0, 1.0)
  assert solve_linear_eq(3, 1, 2, 1) == (1.0, 1.0)
output: def solve_linear_eq(a1, b1, a2, b2):
      D = a1 * b2 - a2 * b1
      Dx = 3 * b2 - 4 * b1
      Dy = a1 * 4 - a2 * 3
      x = Dx / D
      y = Dy / D
      return x, y