After 4 days, Cindy and Kelly will each have a retirement account balance of $3607.
Your two equations are:
3559+12x=y and 3571+9x=y (take the current balance plus the deposit times the number of days (x) is equal to the new balance (y))
Set them equal to each other so you have 3559+12x=3571+9x then solve for x
Get all the numbers on one side and the x's on the other
3571-3559=12x-9x
12=3x
4=x
Now substitute that for x into one of the equations above and solve for y. You can use either one.
3559 + 12(4)= 3559 +48=3607
You can double check with the other equation
3571 + 9(4)=3571+36=3607
Hope that helps