calculus help displacement help

The displacement (in meters) of a particle moving in a straight line is given by s = t² − 5t + 15, where t is measured in seconds.

[3, 4]
m/s

(ii)[3.5, 4]
m/s

(iii)[4, 5]
m/s

(iv)[4, 4.5]
m/s

(b) Find the instantaneous velocity when t = 4.
m/s