An approximation of i.r.o.c. using the difference quotient is just a fancy way of asking you to use the slope formula you've known since 7th grade: m = (y2 - y1)/(x2 - x1) .
If you want to approximate i.r.o.c. at t = 3 with an h = .01 you could use (3 , h(3)) and (3.01 , h(3.01)) as the two points. Or, for a better approximation, "straddle" t = 3 by using (2.995 , h(2.995)) and (3.005 , h(3.005)).