Austin’s gas tank is 1/3 full after he buys 12 gallons of gas it is 7/9 full. How many gallons can Austin’s tank hold

The length of the major axis of the ellipse below is 13.

Let P be the point on the ellipse

Let F1  and F2  are the two focus

The distance of red line is PF1 and the distance of blue line is PF2

In ellipse, the sum of the distance from point P to the focus is equal to the length of major axis

The sum of PF1  and PF2 = length of major axis

So PF1 + PF2 = 13

The sum of red line and blue line = 13