Let x be the total invested in at the 7% account and y the total invested at the 8% account.
Since he has $150 more than the 8% account, it implies that x + y = y + 150 and, therefore, x = $150
Since the total interest is $51, we have:
0.07x + 0.08y = 51
0.07*150 + 0.08y = 51
10.5 + 0.08y = 51
0.08y = 40.5
y = 40.5/0.08 = $506.25
Kevin has $150 at the 7% account and 506.25 at the 8% account.