Algebra

We can probably write some equations with the information we have. 

We have:

$a_n = a_{n - 1} + a_{n - 2}$  Subtracting both sides by a_n-1, we have

$a_{n-2}=a_n-a_{n-1}$

Now, let's plug in numbers we are given in the question. Plugging in 11 gets us $a_9=4-1=3$

Plugging in 10 gets us $a_8=1-3=-2$

Ok, let's see what happens when we plug in 9. 

We get $a_7 = a_9 - a_8$. We already know these two values!

We have $a_7=3-(-2)=5$

Now, we plug in 8. We get $a_6=-2-5=-7$

So -7 is our final answer.

Thanks! :)