Go online and use this good fraction calculator and see if you get the answer:

http://www.calculatorsoup.com/calculators/math/fractions.php

You have to multiply to get a common denominator. Since 67/890 can't simplify down anymore, you have to figure out what can multiply by 34 to get 890. In this case, you can't mulitply 34 to get 890, so you're going to have to multiply them together to get a common denominator. So mulitply the fraction 12/34 by 890 and the fraction 67/890 by 34 (don't forget to multiply the numerators too). Then add the answers together, you should get 6479/15130.