The values are:
- For part (a): \( t = 8 \)
- For part (b): \( s = 0 \)
- For part (c): \( t = \frac{12}{s} \) (where \( s \) cannot be zero)
Thus, the final boxed answers are:
\[
\boxed{t = 8}
\]
\[
\boxed{s = 0}
\]
\[
\boxed{t = \frac{12}{s} \text{ (for } s \neq 0\text{)}}
\]