There is nothing missing. There is 15 new magazines and Carlos reads 6 of them. Compose and equation using M to R to find out how many is left to read. I got the answer wrong using the number 9 as an answer.

Carlos has15new magazines to read.

LetM be the number of magazines he would have left to read after reading R of them.

Compose an equation relatingM to R.

Then use this equation to find the number of magazines he would have left to read after reading 6 of them.