Y-10=9(x+8) its a khan academy question.

Im going to try my hardest to explain this simply because its kinda hard but in 10 years the town would grow by 100% so take 3900 and figure out what 40% of 3900 would be (3900 x .40 = 1560) so add 1560 to the other 100% which is 5460 and then add that to the original population which is now 9360. So the population in 14 years would be 9360. The reason I went into depth is because most teachers want an explanation. and what grade is this???