How do I calculate magnetic declination on a map?

I am trying to teach myself how to obtain the current magnetic declination using the guide on a topographical map. The declination on the map was 8° 40 west in 2009, I am trying to get the current declination in 2019. You re suppose to calculate how many years have gone by, 10 and multiply it by the annual change... show more I am trying to teach myself how to obtain the current magnetic declination using the guide on a topographical map. The declination on the map was 8° 40 west in 2009, I am trying to get the current declination in 2019. You re suppose to calculate how many years have gone by, 10 and multiply it by the annual change which is subtracting by -1each year and I m not sure if I add 1 since I m subtracting a negative number or what. Anyways, it gives me 10° or 600 and you add or subtract that to the previous declination. Either way, it does not give me a declination of 8° west which is the current declination in the area of the map. Can some one help me understand this please?
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