The formula is 2 units HF, 2 units HNO3, and 1 unit H2O.

-- Pick up the HF bottle and click it on the 4-unit flask. Pick up the flask and click it on the 3-unit flask. Pour the one unit remaining in the larger flask into the beaker. Repeat this process to get 2 units of HF. -- Pick up the HNO3 bottle and click it on the 4-unit flask. Pick up the flask and click it on the 3-unit flask. Pour the one unit remaining in the larger flask into the beaker. Repeat this process to get 2 units of HNO3. -- Pick up the H2O pitcher and click it on the 4-unit flask. Pick up the flask and click it on the 3-unit flask. Pour the one unit remaining in the larger flask into the beaker.

Pick up the beaker and click it on the brown bottles at the left. The filled bottle goes into your inventory.