Tangent Lines : The first problem which we're going to study is the tangent line problem.  Before getting into this problem probably it would be best to define a tangent line.

A tangent line to the function f(x) at the instance x = a is a line which just touches the graph of the function at the point in question & is "parallel" (in some way) to the graph at that point. Consider the  graph below.

In this graph the line is a tangent line at the specified point because just it touches the graph at that point and is also "parallel" to the graph at that point.  Similarly, at the second point illustrated, the line does just touch the graph at that point, hence it is not "parallel" to the graph at that point & hence it's not a tangent line to the graph at that point.

At the second point illustrated (the point where the line isn't a tangent line) we will sometimes call the line a secant line.

Now, we've used the word parallel a couple of times and we have to probably be a little careful with it.  Generally we will think of a line & a graph as being parallel at a point if they are both moving in the same direction at that point. So, in the first point above the graph and the line are moving in the same direction and so we will say they are parallel at that point.  At the second point, on the other hand, the line and the graph are not moving in the same direction and so they aren't parallel at that point.