The total return you want is 0.09 * 1600 = $144
Let a be the amount invested at 12% and b be the amount invested at 6%
0.12a + 0.06b = 144 - 44
12a + 6b = 10000
a + b = 1200
Put those 2 equations together and solve for a and b:
12a + 6b = 10000
a + b = 1200
Using elimination, a = $466.67 and b = $733.33
So, out of an additional $1200, invest $466.67 at a rate of 12% and $733.33 at a rate of 6%