Your equation is x²/a² + y²/b² = 1 with a = 5 and b = 6
Have a look to the picture above but exchanging a and b since a < b (or rotate the ellipse of 90°)
c² = b² - a² = 11
Foci are therefore placed at (0, sqrt(11)) and (0,-sqrt(11))
The definition of an ellipse a line. The sum of the distance to each foci of each point is the same.
You know because of the y-intercept that this is equal to