To find the principal amount invested, the formula for simple interest can be rearranged: $P = A / (1 + rt)$, where $A$ is the total amount after interest, $r$ is the rate, and $t$ is the time in years. In this case, $A = 7000$, $r = 0.04$, and $t = 3$, leading to $P = 7000 / (1 + 0.04 \times 3)$. Calculating this gives a principal of N6,250.00.