Wouldn't it just be:

you are adding water which has a 0% acid concentration

since you have 9gallons of acid, you want the total solution to be 36 gallons.

Add 36 - 27 gallons of water = 9 gallons.

Doing it in your table you have conc of water is 100%, but actually it's 0% as you're talking about the acid concentration.

so it would be

so (27+x)/4 = 9

Quantity %age Cal Initially 27 1/3 9 addWater x 0 0 Required Solution x+27 25% 9

x + 27 = 36

x= 9