What is the molarity of a solution that contains...

6 moles of solute in 2 liters of solution

The molarity of the solution of that solute is 3 mol / L.
It's a ratio of moles of solute to 1 L of solution.

molarity =number of mole /volume of a solution in liter.
=6mole/2L
=3M.

6 mol/L x 1mol/2L
therefore it's 3 M