Hi! To answer this question you need to multiply each volume by its respective concentration, then add the two values to find the total moles of Cl-, then divide by the total volume.
(0.915L x 0.349M) + (0.773Lx 0.671M x 2) = 0.3567 mol Cl-
We multiplied the second set of terms by 2 because there are 2 moles of Cl- per mole of MgCl2. This can be observed by writing out the dissociation reaction equations for each salt.
Then 0.3567 mol Cl- / (0.915L+0.773L)=0.804 Molar