In Romeo and Juliet, when Tybalt and Mercutio first begin arguing, what does Benvolio try to get them to do?

In Act III scene i, which charts the fight between Tybalt
and Mercutio and Mercutio's unfortunate demise, it is clear that from the very first
entrance of the Capulets, Benvolio plays the role of peacemaker. His main speech comes
before Romeo enters and Mercutio and Tybalt have begun to insult each
other:

We
talk here in the public haunt of men.

Either withdraw unto
some private place,

Or reason coldly of your
grievances,

Or else depart. Here all eyes gaze on
us.

Note how his speech
attempts to diffuse the explosive situation by reminding both parties of the public gaze
upon them all. Given this audience, it makes sense to withdraw to "some private place"
or to speak rationally and reasonably to each other about the "grievances" that exist
between them.

Thus Benvolio plays the role of rational
observer, trying to stop the potential explosion from breaking out and limiting the
damage.