For the simple interest, the formula is I = P*r*t, where I is interest value, P is the initial amount (deposit); r is the annual rate and "t" is time (in years). To find "I" in your case, substitute P = 840 dollars; r = 0.045 (annual percentage rate as decimal, and t = 4 years. Then calculate I = 840*0.045*4. You may use your calculator/computer.