I will help you on the construction on this problem only so that you will have a chance to learn on your own. Working problems like this by yourself is important to the understanding of problem solving.

So we have three investments, lets say

x is the investment for the rate of 9%

y is the investment for the rate of 10.8%

z is the investment for the rate of 16.2%

Since Susan invests $4900 we know that

1) x + y + z = 4900

Since the total interest of that year is 640.80, we also know that the sum of the generated interest from each investment totals this number. So

2) .09x + .108y + .162z = 640.80

Since we know the amount placed in the third investment is $900 more than the amount placed in the second (I'll assume respectively from lower to higher interest rate), then

3) y = z + 900

Using 1,2,3 solve for the system of linear equations.