In Young's double slit experiment, carried out with light of wavelength $5000~\mathop A\limits^o$, the distance between the slits is $0.3 \mathrm{~mm}$ and the screen is at $200 \mathrm{~cm}$ from the slits. The central maximum is at $x=0 \mathrm{~cm}$. The value of $x$ for third maxima is $\underline{\hspace{2cm}}$$\mathrm{mm}$.