Answer:
Suppose there are 10 tables. Putting 4 chairs at each table requires 4*10 = 40 chairs. Since this is 6 more chairs than he has, he must have 40 - 6 = 34 chairs.
Now suppose we put pairs of table together ("double tables"), so that 6 chairs can fit around each pair. With 10 tables, there will be 5 pairs, and with 6 chairs around each of these, we need 5*6 = 30 chairs. So 4 of the 34 chairs are left over. That's exactly what the problem said, so this is the answer.
Now, you may find that this check provides you with a model for the second equation you need to write! At least, I hope it clarifies what the problem means by "double tables".
Explanation:
this is just my interpritation, please tell me if wrong!!!