How to calculate percent of change of a regression fit


New Member
I have a time series of monthly data over 8 years and I plot a linear fit of the data. The trend shows a decline. Please see the graph.
Now, my task is to calculate 'percent of change' of the linear fit. Namely, I want to know the annual rate of decline in a percentage.
Days of searching on the internet and asking people already. Any help is appreciated.