That's good, you have a good foundation. Since you're only looking for the oxidation number of chromium at this point, let's look at it like this. The polyatomic ion Cr2O7, or dichromate, has a 2- charge. Hence, the charges of the individual ions have to add up to -2. So, we set up an equation, where x is the oxidation number of chromium. In this case, we'll have 2(x) + 7(-2) = -2. See what I did there? I set up an equation so that the individual charges will all add up to -2, so let's solve for x, and when you do that, you'll get +6, which happens to be the answer!