the hard way to do it is to subtract 1,700 from 396,460 again and again till you get 359,060. Then you see how many times you subtracted 1,700 and you would get the answer.
Another way to do this is to use thus information to make and equation with the information you have.
Lets make the number of months equal X
you have 396,460 and you subtract (-) for every month (x) you get 1,700 till you have 359,060
so the equation is:
396,460 - 1,700x = 359,060 (simplify it)
-1,700x = -37,400
x = 22
the answer is 22 months
(easier than subtracting 1,700 from 359,060 22 times)