According to the given information, the sum of the calories of a bag of popcorn and a juice box is 518. Taking x as the number of calories of a bag of popcorn and y as the number of calories of a juice box, we have that:

Then, 2 bags of popcorn (it means 2x) and 6 juice boxes (it means 6y) have a total of 1456 calories, this is:

Use this system of equations and solve the problem, this way:
Solve the first equation for x:

Use this expression and replace for x in the second equation:

Use this value to find the value of x:

It means that there are 413 calories in each bag of popcorn and 105 calories in each juice box.