She keeps 4 marbles in every bag and mark it with 3/4 to represent red marbles in the bag. It means there are 3 Red marbles out of total 4 marbles in the bag.
It says to write another fraction to represent number of red marbles in two bags.
Since one-bag contains 3 Red marbles out of total 4 marbles in the bag.
Then two-bags would contain 2x3 Red marbles out of total 2x4 marbles in the bag i.e. 6 Red marbles out of total 8 marbles in the bag.
So,
is the another fraction to show number of red marbles.