[tex]j) {3}^{x} + {3}^{x - 1} + {3}^{x - 2} = 351 \\ \\ ( {3}^{2} + 3 + 1) \times {3}^{x - 2} = 351 \\ \\ (9 + 3 + 1) \times {3}^{x - 2} = 351 \\ \\ 13 \times {3}^{x - 2} = 351 \\ \\ 13 \times {3}^{x - 2} : 13 = 351 : 13 \\ \\ {3}^{x - 2} = 27 \\ \\ {3}^{x - 2} = {3}^{3} \\ \\ x - 2 = 3 \\ \\ x - 2 + 2 = 3 + 2 \\ \\ x = 3 + 2 = 5 [/tex]
