Every time you add 1 degree of unsaturation to a hydrocarbon, you remove 2 H's from the number of H's that the hydrocarbon would have if it were a completely saturated alkane, aka, the "alkane formula". I think it's better to not memorize the formulas if you do it this way: Alkanes (with only C's and H's) always have 2 more H's than twice the number of carbons. If I had a tricyclic hydrocarbon (without any double bonds) then I'd have 3 degrees of unsaturation. The regular "alkane formula" of the hydrocarbon (aka, if it had the same number of carbons but no degrees of unsaturation, which are rings in this case) is C14H30. ( 2(14) + 2 ). 3 degrees of unsat. means you remove 6 H's from the "alkane formula". So it'd be C14H24.
As for anthracene, its alkane formula would also be C14H30. But this time we have 7 double bonds in addition to 3 rings, so we have 10 degrees of unsat. So we must remove 20 H's from our alkane formula. So 30 H's minus 20 H's gives you 10 H's. So anthracene becomes C14H10.
To find a general formula, just realize that if you start with your alkane formula which is CnH2n+2, you would remove 2 H's for every degree of unsat, so for your tricylic you have 2n+2 -6 which would give you 2n-4. Or CnH2n-4.
