A linear equation has the form y = mx + b where m and b are real numbers. The value of b (you may know this as the y-intercept) is the value of y when x = 0. The value of m (the slope) is the RATE of increase; it tells how much y changes when x increases by 1.
In this problem, x represents the number of years since 2000, so x = 0 represents the year 2000.
Midwest: the value of y begins with 64.4 and increases by 0.3 each year.
y = 64.4 + 0.3x
West: the value of y begins with 63.2 and increases by 1.0 each year.
y = 63.2 + 1.0x