A concrete beam has width $b = 300$ mm and effective depth $d = 500$ mm. The steel reinforcement has area $A_s = 1500$ mm$^2$ and modular ratio $n = 8$. The distance $x$ from the top compressive fiber to the neutral axis satisfies the equation $b \\frac{x^2}{2} - n A_s (d - x) = 0$. What is the value of $x$ (to the nearest mm)?