I know this is right, but I don't really understand it and I need you to explain this to me, please?!

Answer:
x + 2x = 108
Step-by-step explanation:
Let x be the number of oranges sold on Thursday
The number of oranges sold on Friday is twice as many or 2 times x, 2x
The total sold on the two days is 108. Add the number sold on Thursday and Friday together and let it equal to 108
x + 2x = 108