A stone is thrown vertically upward, after't'second it covers a distance of's' metre, where 's'=80t-16t2. It's velocity after 2 second?

You are evidently working in feet and inches assuming this event occurred on earth.

s is the height above the launch point here. 80 ft/s is initial speed up and 16 is g/2

s = 80 t - 16 t^2

the velocity upward is the time derivative of height

v = 80 - 32 t

at t = 2, v = 80 - 64 = 16 feet/second