[dba-Tech] OT Friday - Calculus Problem

Dear Lists:
 
My son has a calc test today and has a problem he doesn't understand.  I
can't help him with it.  Anyone remember how to do this?
 
Position function is s(t) = -4.9t**2 +200
 
which gives the height in meters of an object that is falling from a height
of 200 meters. The velocity at time t = as seconds is given by:
 
lim(t-->a) = ((s(a)-s(t)) / (a-t)
 
FInd the velocity of the object when t=4. The answer in -39.2 m/sec.  How is
that derived?
 
MTIA
 
Rocky