Using what's given, you'd have $2800 = p x 0.04 x 3.5.
We don't know the principle so you'd leave the variable there and convert 4% interest to 0.04. Solve by getting 2800 = p(0.14) then divide each side by 0.14 and get p=20,000. So your principle, or the amount deposited, should be $20000. You can solve by plugging in your answer (20000 x 0.04 x 3.5 = 2800)
Interest = Principal * rate * time
2800 = x * 4% * 3.5
2800/(4% *3.5) = x
20000 = x
