Below is a graph for some moving object.
Estimate its speed at time=4sec

We want the speed at one point along a curve, so we will make a tangent line.

Realize that a range of answers will be acceptable because people will add this line on a little differently.
Now we find the slope of the line, and we're done!

slope=DY/DX=1.7/4=.43m/s

Click here, login, and in chapter 1, solve the 3 "graph problems".  They are image mapped, meaning you must click on the correct area of a picture for credit.