From molarity equation $\begin{aligned}
& \mathrm{M}_1 \mathrm{V}_1+\mathrm{M}_2 \mathrm{V}2=\mathrm{MV}{\text {(total) }} \
& 2 \times \frac{10}{1000}+0.5 \times \frac{200}{1000}=\mathrm{M} \times \frac{210}{1000} \
& 120=\mathrm{M} \times 210 \
& \mathrm{M}=\frac{120}{210}=0.57 \mathrm{~M}
\end{aligned}$