There are already a few replies to this. Just thought I would add a slightly different one. Here is how I would do it:
$$
(-5x^2y^3)^{-2} = \frac{1}{(-5x^2y^3)^2}
= \frac{1}{5^2(x^2)^2(y^3)^2}
= \frac{1}{25x^4y^6}
$$
Just to add that the topics covered are:
Special relativity
http://thephysicsnotes.com/U-Level-Physics-Videos/special-relativity/time%20dilation,%20length%20contraction.html
Statistical Mechanics, Superconductors and Superfluids...
Here are two sets of lectures that I recorded when I taught these courses. Hopefully they will continue to be useful.
http://thephysicsnotes.com/U-Level-Physics-Videos/special-relativity/time%20dilation,%20length%20contraction.html...
I made a set of videos on basic physics concepts:
www.thephysicsnotes.com/A-Level-Physics-Videos/a-level-physics.html
These are meant for 17-18 year old students in Singapore. Let me know what you think.
I have made some videos on basic concepts in physics:
http://thephysicsnotes.com/A-Level-Physics-Videos/a-level-physics.html
These are mainly meant for 17-18 year old students in Singapore. I hope it could also be relevant to students elsewhere.