let x = amount invested at 3%
so, (2600 - x) is invested at 7%
.07 *(2600 - x) + .03 *(x) = 138
182 - .07x + .03x = 138
-.07x + .03x = 138 - 182
-.04x = -44
-.04x / -.04 = -44 / -.04
x = 1,100
so amount invested at 7% is (2600 - 1100) or 1500
consequently amount invested at 3% is 1100
$1,100 invested at 3%
$1,500 invested at 7%