I've been given a table of values and I have to estimate the instantaneous velocity at t = 2002. I can do this by averaging two secant lines, but it wants me to measure the slope of the tangent. Here's the table.

t | N

1996 | 44

1998 | 69

2000 | 109

2002 | 141

2004 | 182

2006 | 233

I really have no idea where to start.