let's say the amounts invested were "a" at 10% or 10/100 and "b" at 8.5% or 8.5/100
the total amount invested was 300,000, so, whatever "a" and "b" are, they add up to that much, thus
a + b = 300,000
10% of a is 10/100 * a or 0.10a and 8.5% of b is 8.5/100 * b or 0.085b
and we know their yield is 28,500 thus
0.10a + 0.085b = 28,500
thus [tex]\begin{array}{lcclll}
&amount&\%&total\\
&-----&-----&-----\\
\textit{invested at 10\%}&a&0.10&0.10a\\
\textit{invested at 8.5\%}&b&0.085&0.085b\\
\end{array}\\\\
-----------------------------\\\\
\begin{cases}
a+b=300,000\implies \boxed{b}=300,000-a\\\\
0.10a+0.085b=28,500\\
----------\\
0.10a+0.085\left( \boxed{300,000-a} \right)=28,500
\end{cases}[/tex]
solve for "a", to see how much was invested at 10%
what about b? well, b = 300000 - a
