Hello Astrid,
To solve this problem, you need to use the compound interest formula.
I have it written out below, then I can explain.
6000(1 + 0.08/2)^(35 x 2)
We are starting with 6000 and multiplying 1 and an additional 8% (0.08). However, since the interest is given twice a year, we divide the 0.08 by 2.
Then, we would raise it to the 35th power because we are compounding it for 35 years. However, it happens twice a year, so we multiply the 35 by 2, for a total of 70.
This will produce the answer you found.
I hope this helps, Good luck!