So since it's decreasing we know that it'll be a negative. It decreases by 7 pine bluffs per year. We can use that equation to model the situation. But let me show you an example to prove it.
Lets say we want to know how many pine bluffs there would be in 3 years y = -7(3) + 6791 y = -21 + 6791
The -21 is the total amount lost in those 3 years, but we have to subtract it from the original amount which was 6791
y = -21 +6792 y = 6770 In three years there would only be 6,770 pine bluffs.
