9x – ……………….. = – 21 has the solution (- 2).

Let 9x-m= -21 has the solution (-2).

Since, $x=-2$ is the solution of the equation.

$\therefore$ $9 \times(-2)-m=-21$

$\Rightarrow$ $-18-m=-21$ [transposing $-21$ to LHS and $-m$ to RHS]

$-18+21=m$

$\Rightarrow$ $m=3$

Hence, $9 x-3=-21$ has the solution $(-2)$.