He gets 3*$400=$<<3*400=1200>>1200 per month
So he gets 1200*12=$<<1200*12=14400>>14,400 per year
So he pays 12*900=$<<12*900=10800>>10800 a year
That means his profit is 14400-10800=$<<14400-10800=3600>>3600 per year
#### 3600