7 gallons of pure acid and 14 gallons of 10% acid
From my research, the image below belongs to the question. From what is seen in the image, there are 2 arcs already drawn with respect to the new vertex. The third arc corresponds to the point where the line DM is made. This line is similar to line BC. So at the point where the second and third arcs would meet, it would be a point similar to a point on line AB.