The unit rate is how many calories are in 1 cup of cereal.
Set up a proportion:

Cross multiply and solve:

There are 100 calories per cup of cereal.
To get the calories in 4 cups of cereal, simply multiply the unit rate of 100cals/cup by 4.

There are 400 calories in 4 cups of cereal.
The correct answer is x= -6 let me know if you want to know how I got that or if you needed to know if it is no solution/ infinite/etc..