Answer
$\$3000$ into the first one ($6\%$) and $\$6000$ into the second one ($7\%$).
Work Step by Step
Let $x$ be the amount he invested into the first account. Then the amount he invested in the second one is $2x$. We also know that the annual interest is $600$. Thus, we have:
$0.06x+0.07(2x)=600\\0.2x=600\\x=3000$
Hence, he invested $\$3000$ into the first one ($6\%$) and $\$3000*2=\$6000$ into the second one ($7\%$).
