#### Answer

$\left( -\infty,1 \right]$

#### Work Step by Step

Using the properties of inequality, the given inequality, $
-0.3x+2.1(x-4)\le-6.6
,$ simplifies to
\begin{array}{l}\require{cancel}
10\left[ -0.3x+2.1(x-4) \right]\le(-6.6)10
\\\\
-3x+21(x-4)\le-66
\\\\
-3x+21x-84\le-66
\\\\
-3x+21x\le-66+84
\\\\
18x\le18
\\\\
x\le\dfrac{18}{18}
\\\\
x\le1
.\end{array}
Hence, the solution is the interval $
\left( -\infty,1 \right]
.$