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?

Let's say that Paco had x salty cookies to begin with. 

Then he ate y of them. 

That leaves him with x-y cookies. 

Now we need to figure out how many y is. We are given that y is 2/3 of x. 

That means y/x = 2/3. Cross multiply to get 3y = 2x. 

That means y = 2/3x. 

Now we can plug this value of y into our original equation: x-y = x-2/3x. 

Simplify to get x/3 = x-2x/3. 

Add 2x/3 to both sides to get 5x/3 = x. 

Divide both sides by 5/3 to get x = 3. 

That means that Paco had 3 salty cookies to begin with and he ate 2 of them.