Hi naspek
Q:Question..
From the graph, estimate the roll-off rate.
how am i going to solve this question?
I suppose, you are able to derive from the picture the gain decrease per decade.
However, my point is another one:
Who ever gave you this task - ask him if he knows what a Butterworth filter is.
If he does not know, tell him that it has as a minimum a conjugate-complex pair of poles, which means it is at least of second order. (Butterworth and first order is nonsense!).