Let the amount invested at 4.5% be x
The amount invested at 7.2% will thus be 2200 - x
The annual interest earned from the 4.5% will be 0.045x
The annual interest earned from the 7.2% will be 0.072(2200 - x)
Since the total annual interest is $144, we have:
144 = 0.045x + 0.072(2200 - x)
144 = 0.045x + 158.4 - 0.072x
144 = 158.4 - 0.027x
0.027x = 158.4 - 144
0.027x = 14.4
x = 14.4 / 0.027
x = 1600 / 3
2200 - x = 5000 / 3
Hence, $(1600 / 3) is invested at 4.5% and $(5000 / 3) is invested at 7.2%