Average velocity = Δs/Δt
This seems to be simply a plug-in.
Δs = s(1 + h) - s(1)
Δt = tf - ti = (1 + h) - 1 = h
where s = 6t2 + 4.
For part (i), h = 0.1, so
s(1) = 10, s(1 + h) = 6*1.12 + 4 = 11.26, and so
Δs = 11.26 - 10 = 1.26
avg v = 1.26/0.1 = 12.6 m/s
Parts (ii) and (iii) follow the same pattern