Answer
$\left\{ x|x\le4.475 \right\}
\text{ or }
\left( -\infty, 4.475 \right]
$
Work Step by Step
Using the properties of inequality, the given inequality, $
0.4x+5.7\le2.6-3(1.2x-7)
,$ is equivalent to
\begin{array}{l}\require{cancel}
0.4x+5.7\le2.6-3.6x+21
\\\\
0.4x+3.6x\le2.6+21-5.7
\\\\
4x\le17.9
\\\\
x\le\dfrac{17.9}{4}
\\\\
x\le4.475
.\end{array}
Hence, the solution is $
\left\{ x|x\le4.475 \right\}
\text{ or }
\left( -\infty, 4.475 \right]
.$