解：?$(1)$?將函數(shù)?$y = 2x - 3$?的圖象向上平移?$6$?個單位長度
∴所得直線對應(yīng)的函數(shù)表達式為?$y = 2x - 3 + 6 = 2x + 3$?
?$ (2)$?設(shè)平移后的函數(shù)表達式為?$y = 2x - 3 + m$?
∵圖象經(jīng)過點?$P(0，$??$5)$?
∴?$5 = 2×0 - 3 + m，$?解得?$m = 8$?
?$ $?即圖象向上平移?$8$?個單位長度可以經(jīng)過點?$P(0，$??$5)$?
?$ (3)$?對于?$y = 2x + 3$?
?$ $?關(guān)于?$x$?軸對稱：?$-y = 2x + 3，$?即?$y = -2x - 3；$?
?$ $?關(guān)于?$y$?軸對稱：?$y = 2(-x) + 3 = -2x + 3$?
