Whichever process suits you (or do it 2 or three different ways, just as a check).
As you've started, you can PV the two given amounts back to today. Then compound (FV) that single amount forward for six periods (three years, with semi-annual compounding).
Or (just as an example), you can FV the 850 forward for 2 periods, and discount the 600 back for 4 periods, and sum. Identical results. (Actually, I think this particular approach is more consistent with the instructions to use the t=3 point as the 'focal' date.)
There's actually no limit to the ways to arrive at the correct answer. For example, if you discount a given amount back 5 periods, and then forward 3 periods, it's the same result as if you'd discounted it back 2 periods, or back 17 and forward 15, or.....you get the drift. While it's a waste of time to actually do all that, the intuitive understanding of the underlying principal can be real helpful in visualizing the problem before you hit the calculator.