1. **State the problem:** We need to find the principal amount invested in the bank to get a profit (simple interest) of 3500 at an interest rate of 7% per annum for 4 years. 2. **Recall the formula for simple interest:** $$SI = \frac{P \times R \times T}{100}$$ where $SI$ is the simple interest, $P$ is the principal, $R$ is the rate of interest per annum, and $T$ is the time in years. 3. **Substitute the known values:** $$3500 = \frac{P \times 7 \times 4}{100}$$ 4. **Simplify the equation:** $$3500 = \frac{28P}{100}$$ 5. **Solve for $P$:** $$P = \frac{3500 \times 100}{28} = \frac{350000}{28} = 12500$$ 6. **Conclusion:** The amount invested in the bank is $12500$.