to find how much the population increased in the period of four years, subtract 6200 from 10800. this gives you 4600. because 4+4 is 8, we know that this is how much the population will have increased by after 8 years. Add 4600 to 10800 to get 15400.
if this is exponential growth, the answer will be different but because of the way you phrased the question I am assuming linear growth