The table shows the position of a cyclist.
t (seconds) 0 1 2 3 4 5
s (meters) 0 1.6 5.5 10.6 17.1 24.9
(a) find the average velocity for each time period.
(i) [1, 3]

4.5

correct: your answer is correct.
m/s

(ii) [2, 3]

5.1

correct: your answer is correct.
m/s

(iii) [3, 5]

7.15

correct: your answer is correct.
m/s

(iv) [3, 4]

6.5

correct: your answer is correct.
m/s

(b) estimate the instantaneous velocity when t = 3.

5.6

incorrect: your answer is incorrect.
m/s

the anwer 
the main formula concerning such a problem is as follow:
v= Delta x  / Delta t =Δx / Δt = xf  - xi / tf - ti
f = final value, i=initial value

Find the average velocity for each time period.
when t is in [1, 3]  
so 
v=  xf  - xi / tf - ti = 10.6 -1 .6 =9 / 3 - 1 =9 / 2 = 4.5
your answer is correct

when t is in [2, 3] 

v = 10.6 - 5.5 / 3 - 2 = 5.1 m/s  

your answer is correct

when t is in [3, 5] 

v = 24.9 - 10.6 / 5-3 = 7.15 m /s
your answer is correct

when t is in [3, 4] 
 v = 17.1 - 10.6 / 4 -3 =  6.5 m/s
your answer is correct

the instantaneous velocity when t =3

v = Δx / Δt = 10.6 / 3 = 3.53 m/s 

What’s the equation you are trying to solve to get x?