Answer:
At the beginning, there were 2,678.26 grams of sugar in the container.
Explanation:
Since Lisa used 880g of a container of sugar to bake a cake and 1/10 of the creaming sugar to make cookies, and she then had 3/7 of the container of sugar left, to determine how much sugar was in the container at first, the following calculation must be performed: 
 
880 + 1 / 10X = 3 / 7X 
880 + 0.1X = 0.4285X 
880 = 0.4285X - 0.1X 
880 = 0.3285X 
880 / 0.3285 = X 
2,678.26 = X 
 
Therefore, at the beginning there were 2,678.26 grams of sugar in the container.