what does (ds/dt)² (d²s/dt²⁾ represent

ds/dt = velocity

so

(ds/dt)² = velocity²

and

d(velocity)/dt = acceleration

d (ds/dt) / dt = acceleration

d²s/dt² = acceleration

For understanding the logic:

First, go through this link: 

https://www.meritnation.com/ask-answer/question/what-is-a-slope-in-a-graph-i-have-studied-distance-time/motion/5107314

Then,

Please try to understand the topic of differentiation as given the ncert math textbook, just the basics. Then you will be able to understand the concept used. It is highly important that you understand this so that you may get good marks.

Feel free to ask if you have any doubts.