\[\boxed{\text{1141.\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[x = 4,8.\]
\[\lbrack 3x\rbrack = \lbrack 14,4\rbrack = 14.\]
\[\lbrack - 3x\rbrack = \lbrack - 14,4\rbrack = - 15.\]
\[\left\{ 3x \right\} = 14,4 - 14 = 0,4;\]
\[\left\{ - 3x \right\} = - 14,4 + 15 = 0,6.\]