Someone me.

andrea wants to deposit money into a bank account that earns 4% simple interest use the formula l=prt to find that amount of money that she would deposit so that she earns $2800 after 3.5 years show and explain your work.

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