my_list = [[31, 27, 10], [8, 11, 12], [11, 12, 13], [6, 9, 10]]

print("The list is :")
print(my_list)

my_result = []

for row in my_list:
   temp = True
   for index in range(0, len(row) - 1):

      if row[index + 1] - row[index] != row[1] - row[0]:
         temp = False
         break

   if temp :
      my_result.append(row)

print("The resultant list is :")
print(my_result)

The list is :
[[31, 27, 10], [8, 11, 12], [11, 12, 13], [6, 9, 10]]
The resultant list is :
[[11, 12, 13]]