Paco had 26 salty cookies and 17 sweet cookies. He ate 14 sweet cookies and 9 salty cookies. How many salty cookies did Paco have left?
There are 26 salty cookies and 17 sweet cookies, so we can write:

salty = 26
sweet = 17

Paco eats 14 sweet cookies and 9 salty cookies, so we can write:

salty' = salty - 9
sweet' = sweet - 14

We are looking for the number of salty cookies left, so we can write:

salty' = ?

salty' = salty - 9
salty' = 26 - 9
salty' = 17