Many of the skills in sixth grade math are more complicated than those learned in elementary school. Basic computation forms a foundation for work with fractions, decimals, ratios and more. Problems require a good command of all four operations plus knowledge of concepts like properties, order of operations, and positive/negative numbers. Many new ideas are expressed as standard formulas using letter names for variables, such as ½ bh for the area of a triangle.
If you are having trouble recalling and applying formulas or procedures, start a math notebook. This is separate from the binder required by the teacher. You will keep it permanently and add to it year after year. In your notebook, write down specific, step-by-step directions for using each formula or for going through a procedure. Put the directions in your own words so they are easy to understand and remember. Make note of when to use the idea, how to determine which number matches which variable and the order of the steps to accomplish.