Answer
$ t\lt 9$.
The graph is shown below.
Work Step by Step
The given inequality is
$\Rightarrow t-21\lt -12$
Add $21$ to each side.
$\Rightarrow t-21+21\lt -12+21$
Simplify.
$\Rightarrow t\lt 9$
Hence, the solution is $ t\lt 9$.
Use an open circle because $9$ is not a solution.
Shade the number line on the left side.
