LVAI

How do you solve for the rate constant in second order reaction? With the formula rate constant is equal to the rate over concentration. I read that the rate constant differs but in a first order reaction the rate constant is the same/ constant in every experiment but in asecond order, it changes. 

I am also confused. In the book there is a question that said, "How can you determine how large a triangle to draw when determining the slope of a curve at a particular point of time" i cant seem to figure it out. Based from what i unserstood, instantaneous rate means in a small interval of time, that you have to pick point close to your point of time which was t = 600 but how come the tangent line stopped at t= 400s and not at t= 500s