If we are talking about simple interest (and not compound interest), then
i = P*r*t
6 = P*0.015*2
6 = P*0.03
0.03P = 6
P = 6/0.03
P = 200
The principal is $200.
This means that if you deposit/invest $200 at an annual interest rate of 1.5% for 2 years, then you'll earn $6 in simple interest.
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note: 1.5% = 1.5/100 = 0.015