Given the equations:
- \( 3t + 2g = 51 \)
- \( t = 21 - g \)
And that \( g = 12 \), we can substitute \( g \) into the second equation to find the value of \( t \).
Substituting \( g = 12 \) into the equation for \( t \):
\[ t = 21 - g \] \[ t = 21 - 12 \] \[ t = 9 \]
Thus, the value of \( t \) is:
\[ t = 9 \]