I could not understand one step in correctness proof of paper titled "A tree-based CP-ABE scheme with hidden policy supporting secure data sharing in cloud computing" authored by Runhua Xu, Yang Wang, Bo Lang.
Please refer the screenshot attached. Specifically, I could not understand the move from step 3 to step 4 in which they replaced one of the denominator (highlighted in screenshot).
I will be very grateful if someone can help me understand this move. Many thanks in advance.