for a) , average velocity means distance/time so find [s(3)-s(0)]/(3-0). And displacement will just be distance, i.e. s(3) - s(0)

for b) they ask you for absolute value of v(t) at the endpoints. and also a(t)

Don't forget that: s'(t) = v(t) ; s''(t) = a(t) = v'(t).