displacement is given by equation
[tex]d = 18t^2[/tex]
now at t = 5 s the position is
[tex]d_1 = 18 *5^2 = 450 m[/tex]
similarly position at t = 9 s
[tex]d_2 = 18*9^2 = 1458 m[/tex]
so the displacement of object in given interval of time will be
[tex]d = 1458 - 450 = 1008 m[/tex]
time interval
[tex]\delta t = 9 - 5 = 4 s[/tex]
now the average velocity will be given as
[tex]v = \frac{\delta x}{\delta t}[/tex]
[tex]v = \frac{1008}{4} = 252 m/s[/tex]
so its average speed is 252 m/s