How Do You Find Instantaneous Velocity at 1s?

[hackett5]

The position versus time for a certain object moving along the x-axis is shown. The object’s initial position is −2 m. Find the instantaneous velocity at 1s.
http://img23.imageshack.us/img23/5285/phy1w.jpg

Using points (0,-2) and (2,7) I found the equation of the line to be =7/2x-2. Since this is a straight line I thought the tangent line would have the same slope. So, at 1s the instantaneous velocity would be 7/2, but this is not the correct answer. Where am I going wrong?

 

[rl.bhat]

Slope of the line = (y2 - y1)/(x2 - x1).
From 0 to 2 seconds this will be the velocity of the particle.

 

[hackett5]

Thanks for pointing out my now obvious mistake! I didn't even think to check to see if I had gotten the slope correct, I just assumed my understanding of instantaneous velocity was wrong.