For simple interest, the amount of interest (I) is calculated from the principal (P), the annual rate (r) and the number of years (t) using the formula
... I = P·r·t
Fill in the given numbers, and solve for t.
... 84 = 600·0.07·t . . . of course, you know that 7% = 7/100 = 0.07
... 84 = 42t . . . . . . . . simplify
... 84/42 = t = 2 . . . . divide by the coefficient of t
