On Sasha's desk there are c green notebooks. The number of red notebooks on her desk is 6 less than number of green notebooks. How many notebooks are there on Sasha's desk if all of them are either green or red?

To represent how many total notebooks are on Sasha's desk, you can represent the number of green notebooks and red notebooks in terms of c, the number of green notebooks.

red = c-6

Green=c

c + (c-6) or 2c - 6 represents the number of total notebooks on Sasha's desk.