This is what I call a fake function problem. It looks as though it's a topic you've never studied, so it's tempting to skip it, but resist the urge. Nobody else has studied it either. They've simply created a symbol which represents a set of instructions. All you need to do is follow the instructions.
In this case, ⊕ means "double the number to the left and then subtract the number on the right." Let's do that with (3 ⊕ 1) ⊕ 2 and 6 ⊕ 3. Firstly, they often nest these fake functions inside of parenthesis. But all you need to do is perform the function inside the parenthesis first, then apply the result to the outside function.
So, 3 ⊕ 1 comes first. Just double 3 and subtract 1, giving us 5. Now since (3 ⊕ 1) = 5, we can plug in 5 in the place of the inside function, giving us 5 ⊕ 2. Repeating the process, let's double 5 and subtract 2, giving us 8.
Now we'll do 6 ⊕ 3: double 6 and subtract 3, giving us 9.
Finally, they want the absolute difference of these two values, or 8  9, which equals 1.
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