Let the amount invested in the account with 11% rate be $p
For the first account:
Amount invested = $p
Interate rate = 11%
The interest gained on this account = 11% of p = 11/100 x p = $0.11p
For the second account, we are told he has $900 more. Thus
Amount invested = $(p + 900)
Interest rate = 15%
The interest gained on this account = 15% of (p + 900) = $0.15(p+900)
We are told the total interest is $213,
This means
$0.11p + $0.15(p+900) = 213
0.11p + 0.15(p + 900) = 213
0.11p + 0.15p + 135 = 213
0.26p + 135 = 213
0.26p = 213 - 135
0.26p = 78
Divide both sides by 0.26
p = 78/0.26
p = 300
Hence, the amount invested in the first account is $300
While the amount invested in the second account is
(p + 900) = 300 + 900 = $1200