input: Write a python function to subtract two matrices of the same size.
  assert subtractMatrix([[1, 2], [3, 4]], [[1, 1], [1, 1]]) == [[0, 1], [2, 3]]
  assert subtractMatrix([[5, 6], [7, 8]], [[1, 2], [3, 4]]) == [[4, 4], [4, 4]]
  assert subtractMatrix([[1, 1, 1], [1, 1, 1]], [[0, 0, 0], [0, 0, 0]]) == [[1, 1, 1], [1, 1, 1]]
output: def subtractMatrix(mat1, mat2):
      return [[mat1[i][j] - mat2[i][j] for j in range(len(mat1[0]))] for i in range(len(mat1))]