While this question may look complicated, it is surprisingly simple to complete. I will break it down word by word, and show the full process to solve.
I like to start with the last sentence of a problem to find out what I am looking for. In this problem the last sentence is "How many grams of oxygen are there?" This is straightforward, and tells us that we are looking for oxygen in the final product of something.
The second and third sentences are "It contains 1.50 g of carbon and 2.00 g of oxygen. Another sample of ascorbic acid isolated from citrus fruits contains 6.55 g of carbon." This tells us that a sample of ascorbic acid contains 1.50 g of carbon and 2.00 g of oxygen, and the carbon content of a secondary sample. Clearly, because we are looking for the oxygen content, this is the important info to know.
The first sentence, "A sample of ascorbic acid (Vitamin C) is synthesized in the laboratory," tells us nothing at all. It sets up a 'setting' for the math problem, and puts some story into it, but when you read this, you will learn nothing that helps you solve the problem.
In the end we are left with the quantity of components in ascorbic acid and a single component from a second sample, and are asked to find the missing quantity. As a math problem, you would write
where x is the missing quantity. Solving, you get 8.7333(...) g oxygen.