In print, it has already explained the convention that has been used to check an addition answer. It says, "Check your answer by drawing two lines above the problem and adding up. Write your check above the two lines".
You could spur in the two lines in braille, or you could explain in a TN what convention you are going to use in braille to for indicating the two lines.
Depending on whether or not that section of the book has taught carried numbers, you could [u]possibly[/u] use dots 2356 to represent the double line. If this symbol has already been used to indicate carried numbers, it may be confusing to the reader to see it used that way here. It is difficult to give suggestions based on one page of print, and not knowing the grade level. If you chose this option you would need to make it very clear in your transcriber's note that it applies only to this particular set of questions. My first choice would be to use spur lines.