Answer
$\frac{v_f-v_i}{a}, \ \ 9.90
.$
Work Step by Step
To solve the given equation for $t$, we move out the term $v_i$ and then divide both sides by $a$ as follows
$$
v_f=v_i+at\Longrightarrow v_f-v_i=at \Longrightarrow t=\frac{v_f-v_i}{a}
.$$
Now, by substitution, we have
$$
t=\frac{v_f-v_i}{a}=\frac{193.1-14.9}{18.0}=9.90
.$$
