Help with this ellipse equation

Ok, my teacher did an example of the problem which is:
Teacher example: Find the standard form of the equation of the ellipse with the given characteristics and center at the origin. Foci: (±5, 0); major axis of length 16

Solution(answer): 1/64 times x^2 + 1/39 times y^2 =1

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A similar problem that I cant figure it out:
Find the standard form of the equation of the ellipse with the given characteristics and center at the origin.
Foci: (±3, 0); major axis of length 12

I don't get how my teacher did this type of problem. Please show me how you got the answer.

Thanks

If you plot the foci, you'll see that one is directly to the right of the other. In other words, they define a horizontal line. For a horizontal ellipse:
     (x−h)²/a² + (y−k)²/b² = 1
     center (h, k)
     a ≥ b > 0
     length of major axis = 2a
     foci (h±c, k) where c²=a²−b²

Apply what your data.
The center (h, k) is exactly halfway between foci, so
h = k = 0

length of major axis 2a = 16
a = 8

foci (h±c, k) = (-5, 0) and (5, 0)
c = 5

c² = a² - b²
5² = 8² - b²
b² = 39

The equation becomes
     x²/64 + y²/39 = 1
:::::
Similarly, foci = (-3, 0) and (3, 0) tells you that the ellipse is horizontal, the center is (0, 0), and c = 3.

Length of major axis = 12, so a = 6.

Use the equation c² = a² - b² to find the value of b².

Now you know all the variables in the equation.