Answer : A = 4a2b6 -5b4 + 3ab3
In descending order of b powers means the term whose b has the greatest power comes first, followed by the nxt and the one with the lowest power be the last in that order
From is the powers of b in the expression are 3,6,4
Arranging this in descending (biggest to smallest)
= 6,4,3
Hence the expression in descending order of the powers of b is
4a2b6 -5b4 + 3ab3