解：?$(1)$?∵點(diǎn)?$P $?在?$y$?軸上，∴其橫坐標(biāo)為?$0$?
即?$2x - 1 = 0，$?解得?$x = \frac 12$?
?$(2)$?∵點(diǎn)?$P $?在第一象限，∴其橫、縱坐標(biāo)均為正數(shù)
即?$2x - 1 ＞ 0$?且?$3x ＞ 0，$?解得?$x ＞ \frac 12$?
又∵點(diǎn)?$P $?到兩坐標(biāo)軸的距離之和為?$9$?
∴?$(2x - 1) + 3x = 9，$?即?$5x - 1 = 9，$?解得?$x = 2$?
則點(diǎn)?$P $?的橫坐標(biāo)為?$2x - 1 = 2×2 - 1 = 3，$?縱坐標(biāo)為?$3x = 3×2 = 6$?
∴點(diǎn)?$P $?的坐標(biāo)為?$(3，$??$6)$?
