Question

you said to us, "how do you solve this sum?"(convert indirect speech)​

Answer 1

$\sf{\huge{Indirect\: Speech}}$

You asked us how we solved that sum.

Answer 2

You asked us how did we solve that sum.

The given question is in the direct speech where the speech is said by someone.

On the other hand for the indirect speech there is a reference to the speech that is reported.

Indirect speech means that the content of the speech is expressed indirectly through questions or utterances that are reported.