Calculus question...a particle moves with a displacement ..

A particle moves with a displacement given by the expression
y=20t - 4.9t^2
find the instantaneous velocity at time t=3 seconds

v(t) = dy/dt = 20 - 9.8t

=> v(3) = 20 - 9.8(3) = 20 - 29.4

i.e. the velocity is -9.4..........or 9.4 in the opposite direction to motion.

:)>

Velocity is differential of displacement wrt time...

v=dy/dx=20-9.8t.

Sticking t=3 into that gives, v=20-29.6=-9.6.