Find a common denominator, its 12
3 6 9 12
4 8 12
Since it took four to get to 12 while counting by 3's you multiply 2/3 by 4 and get 8/12. Multiply 1/4 by 3 and get 3/12, add them together so in total you used 11/12 cups of flour.
Multiply 8 by 12 to get 96 as your numerator for the number of total cups you have then subtract. 96/12-11/12=85/12. I'm sure you can figure out the simplify part out
