$${x = h_m = - v_{0x} \frac{v_{0x}}{g_x} + \frac{g_x \frac{v_{0x}^2}{2g_x}}{2} = -\frac{v_{0x}^2}{g_x} + \frac{v_{0x}^2}{2 g_x} = -\frac{v_{0x}^2}{2g_x}}$$