The slope of a tangent line at a point

Slope of a Tangent Line to a curve at a point

Click on point b and slowly move it toward point a. What value does the slope of the secant lines approach as b approaches a from the left? Is this the same value as b approaches a from the right? Notice the black line is called the tangent line to the curve at point a. What is your estimate of the slope of the tangent line? Explain