Answer:
The amount invested at 4% was $600
The amount invested at 7.5% was $3,600
Explanation:
Let
x ----> the amount invested at 4%
$4,200-x -----> amount invested at 7.5%
we know that
The total interest earned is equal to the interest earned by the amount invested at 4% plus the interest earned by the amount invested at 7.5%
Remember that



so
The linear equation that represent this situation is

Solve for x





therefore
The amount invested at 4% was $600
The amount invested at 7.5% was $3,600