It isn't stated, but I'm assuming it is a constant decrease each year (so the population is following a line -- it's linear).

If you lose 4,000 in 5 years, that's the same as 800 each year.

-4000/5 = -800

If this trend continues for 6 more years, that will be a further decrease of 4800 in population:

-800 * 6 = -4800

So the final population will be:

210,000 - 4,800 = 205,200

A more generic way to solve this would be to write the equation of a line. Let's call the start as year 0 when the population is 214,000. For every year after that, the population decreases 800.

So the population in year 'x' will be modeled by this linear equation:

y = 214,000 - 800x

As a double-check, let's find the population for year 5 (should be 210,000) and then for year 11 (the year from the question).

Year 5:

y = 214,000 - 800(5)

y = 214,000 - 4,000

y = 210,000

Year 11:

y = 214,000 - 800(11)

y = 214,000 - 8,800

y = 205,200

Answer:

205,200 (assuming a linear model)