What ideas have you had so far?
You might just get the right answer for the wrong reasons. *slaps your wrist*
You don't know that and commute! You can't just slip past the like that. However, you can left- and right-multiply by inverses, if you know they exist. In this case, you've been given , so you know that exists. How about this first step:
What else could you do?
A few comments:
1. What is No need to introduce another variable if you don't need it.
2. The notation should really be Otherwise, someone might think you're raising to the power
3. You've got the order wrong. You need to right-multiply by That is, you've got to do this (picking up from where I left off in my last post):
How can you compute the RHS here?