Answer:


Step-by-step explanation:
Use the Quadratic formula:

You can identify that, in this case:

Now you need to substitute these values into the formula:


Remember that:

Therefore,rewriting and simplifying, you get:



Then, you get the following roots:


Answer: 4
Step -by-step :
7+7+7=21
5+12+a=
17+a
21-17=4
You would need to add 360L of the 20% ethanol solution.