so... the amounts Gary invested are... hmm say "a" and "b", at 11% and 12% respectively
we know he invested the whole 70,000, so whatever "a" and "b" are,
a + b = 70,000
what's 11% of a? well 11/100 * a or 0.11a <-- that's how much he earned in interests from that investment
what's 12% of b? well 12/100 * b, or 0.12b <-- that's how much he earned in interests from that investment
whatever 0.11a and 0.12b are, we know they total 7,900
so..... 0.11a + 0.12b = 7,900
thus [tex]\bf \begin{cases}
a+b=70,000\implies \boxed{a}=70,000-b
\\\\
0.11a+0.12b=7,900\\
--------------\\
(\boxed{70,000-b})+0.12b=7,900
\end{cases}[/tex]
solve for "b", to see how much he invested at 12%
what about the 11% investment? well a = 70,000-b
