It's easy bro. Once you've gotten , just put it to get critical points. Then show that on either side of the critical point.
I hope that helps.
A *strictly* decreasing function has its derivative less than zero everywhere.
A function which is just "decreasing" has its derivative less than or equal to zero everywhere.
That is, its gradient is allowed to be zero at certain points, just never positive.
You've been asked to show that this function is decreasing, not that it is strictly decreasing.
It's a subtle point but one worth taking on board.