Instantaneous speed

Q. A particle's motion is given by $s=a(t+b(e^{(-\frac{t}{b})}-1))$, where ($s$ is in metre, $a$ and $b$ are constants and $t$ is in sec). What will be the instantaneous speed of the particle at $t=\infty$

1. $a$
2. $b$
3. $\infty$
4. $0$

Q. The displacement of the particle is given by $x=3t^3+6t^2+2t+5$ in $\text{m}$. The instantaneous speed of the particle at $4\text{sec}$ is

1. $194\text{m/s}$
2. $180\text{m/s}$
3. $200\text{m/s}$
4. $156\text{m/s}$