Answer
See the detailed answer below.
Work Step by Step
a) From the given graph, it is obvious that the leading edge of graph (b) is the leading edge of the right graph in Figure (a).
We can see that it starts at $x=3$ and ends at $x=7$ while its speed is 1 m/s. So this snapshot is taken at $t=4$ s.
$$t=\color{red}{\bf 4}\;\rm s$$
---
b) The history graph at $x=5$ m from 0 s to 6 s:
