Answer
$a=-0.6$.
Work Step by Step
The given equation is
$\Rightarrow -21a+28a-6=-10.2$
Add like terms.
$\Rightarrow 7a-6=-10.2$
Add $6$ to both sides.
$\Rightarrow 7a-6+6=-10.2+6$
Simplify.
$\Rightarrow 7a=-4.2$
Divide both sides by $7$.
$\Rightarrow \frac{7a}{7}=\frac{-4.2}{7}$
Simplify.
$\Rightarrow a=-0.6$.
Check:-
$\Rightarrow -21a+28a-6=-10.2$
$\Rightarrow 7a-6=-10.2$
$\Rightarrow 7(-0.6)-6=-10.2$
$\Rightarrow -4.2-6=-10.2$
$\Rightarrow -10.2=-10.2$
True.
Hence, the solution is $a=-0.6$.
