#### Answer

$t=-12
$

#### Work Step by Step

Using the properties of equality, the solution to the given equation, $
9-t=21
$, is
\begin{array}{l}
-t=21-9
\\\\
-t=12
\\\\
t=\dfrac{12}{-1}
\\\\
t=-12
.\end{array}
Checking:
\begin{array}{l}
9-(-12)=21
\\\\
9+12=21
\\\\
21=21
\text{ (TRUE)}
.\end{array}
Hence, the solution is $
t=-12
.$