I'm having trouble with this one...I understand others have posted this it seems. However, I don't understand those answers/others some incorrect. I first tried thinking of the different ways this could happen, arranging each case i.e.
case 1: 2 followed by string of n-1 containing 00
case 2: 2 followed by string of n-1 containing 11
case 3: 10 followed by string of n-2 containing 11
case 4: 01 followed by string of n-2 containing 00
case 5: 011 followed by any string of n-3
case 6: 100 followed by any string of n-3
case 6: 00 followed by any string of length n-2
case 7: 11 followed by any string of length n-2
however, not convinced this is correct since im fairly certain there is some overlap/some possiblities im missing.
im assuming it would be a better idea to find ones that DO NOT contain "00" or "11" and subtract that from the total possible 3^n, but im not sure where to start...