Step-by-step explanation:
the standard unit used here is 1 second.
constant acceleration creates an arithmetic sequence of speeds. in this case, updated every second by a constant term.
that constant term (d) is in our case here 1m/s, which is added every second (hence the 1m/s² for the acceleration).
so,
a0 (initial speed) = 5 m/s
a1 (after 1 second) = 5 + 1 = 6 m/s
a2 (after 2 seconds) = a2 + 1 = a1 + 1 + 1 = 5 +2 = 7 m/s
...
and we see
an = a0 + n×d
therefore the speed (or velocity) after 4 seconds of that constant acceleration is
a4 = a0 + 4×d = 5 + 4×1 = 5 + 4 = 9 m/s