Finding the concentration of a solution
The molar mass $M$ of $\ce{NaCl}$ is $58.44\ \mathrm{\frac{g}{mol}}$ indeed:

$\displaystyle\text{molar mass}(\ce{NaCl}) = \frac{\text{mass}}{\text{amount of substance}} = 58.44\ \mathrm{\frac{g}{mol}}$

$\displaystyle\Rightarrow \text{amount of substance} = \frac{\text{mass}}{\text{molar mass}}=\frac{10.0\ \mathrm g}{58.44\ \mathrm{\frac{g}{mol}}} \approx 0.171\ \mathrm{mol}$

$\displaystyle\text{concentration} = \frac{\text{amount of substance}}{\text{volume}}=\frac{0.171\ \mathrm{mol}}{0.75\ \mathrm L}=0.228\ \mathrm{\frac{mol}{L}}$

Assumptions: Adding $\ce{NaCl}$ to these $750\ \mathrm{ml}$ does NOT change the volume noteworthy.