she invested.. .say "a" amount on the 4% and "b" amount on the 5% one
now, she lost the 4% one or "a", so... that'd be a negative amount
now, we know both investments added together add up to 9000
and we also know the "net" receipts for the yield was 351, that simply means that whatever "b" and "a" yielded, ended up as 351
so.. what's 4% of a? well (4/100) * a or 0.04a
what's 5% of b? well (5/100) * b or 0.05b
since "a" was a loss(negative), then we know the net yield is => b - a = 351
thus [tex]\bf \begin{cases}
b+a=9000\implies \boxed{a}=9000-b\\\\
0.05b - 0.04a = 351\\
----------\\
0.05b - 0.04\left(\boxed{ 9000-b }\right) = 351
\end{cases}[/tex]
solve for "b", to see how much was invested at 5%
what about a? well, a = 9000 - b
