Line, Tanget line, derivative

https://www.khanacademy.org/math/ap-calculus-ab/ab-differentiation-1-new/ab-2-1/v/derivative-as-slope-of-tangent-line

A tangent line is a straight line that touches a function at only one point. ... The tangent line represents the instantaneous rate of change of the function at that one point. The slope of the tangent line at a point on the function is equal to the derivative of the function at the same point 

https://clas.sa.ucsb.edu/staff/lee/secant,%20tangent,%20and%20derivatives.htm

When I think of a tangent line, I am kind of overthinking. I didn't notice y = c, it must be a horizontal line. So I try to draw a tangent line at any point. It is difficult. Then, I notice it is y = c. Everything becomes easy. 

如果想成直线该如何做