A particle is moving along the curve $y=\dfrac{e^x-e^{-x}}{e^x+e^{-x}}$. As the particle passes through the point $x=\ln2$, its x-coordinate increases at a rate of 5cm/sec. How fast is the slope of the tangent line to the curve $y=\dfrac{e^x-e^{-x}}{e^x+e^{-x}}$ at the particle changing at that instant?
This is an exercise question of my brother, but I did not quite understand the English. Am I supposed to find $d^2y/dx^2$ at that point or just calculate the first derivative? What does "how fast does the slope of the graph of the function change?" mean? Should I bring in a new time variable $t$, also?