To add fractions, you have to have like denominators.
2 of your fractions have like denominators so you don't have to touch them. With 3/4, you have to find what number times 4 equals 8.
That number is two, what ever you do to the bottom you have to do to the top. So 3 * 2 is 6 and 4 * 2 is 8.
Your new fraction is now 6/8.
Regularly add your fractions together 6/8+3/8. That gets you 9/8
Then subtract the amount that you gave away. That would be 9/8 - 5/8 = 4/8
4/8 simplified is 1/2.
Now Luna only has 1/2 of a glass of juice