Answer
$\$9600$
Work Step by Step
Let $x$ be the amount she invested additionally. Then the sum of the interest gained after investing $6000$ at $3\%$ and $x$ at $1.25\%$ is $300$. Then our equation is:
$6000\cdot0.03+0.0125x=300\\0.0125x=120\\x=\$9600$