To find out how much baking soda Lisa should add to 9 lbs. of flour, we can set up a ratio based on the amount of baking soda used for 6 lbs. of flour.
Lisa adds 2 teaspoons of baking soda for 6 pounds of flour.
To find the number of teaspoons of baking soda needed per pound of flour, we calculate:
\[ \text{Baking soda per pound} = \frac{2 \text{ tsp}}{6 \text{ lbs}} = \frac{1}{3} \text{ tsp/lb} \]
Now, for 9 lbs. of flour, we multiply the amount of baking soda per pound by 9:
\[ \text{Baking soda for 9 lbs} = 9 \text{ lbs} \times \frac{1}{3} \text{ tsp/lb} = 3 \text{ tsp} \]
So, Lisa should add 3 tsp of baking soda to 9 lbs. of flour.
Therefore, the answer is 3 tsp.