What is “a, b and c” in this ellipse?

x^2/9+y^2/4=1

comparing it with the standard form

x^2 /a^2 + y^2/b^ = 1

a^2 = 9 ----> a = 3
b^2 = 4 ---> b = 2

and with an ellipse with major axis along the x-axis:
b^2 + c^2 = a^2
4 + c^2 = 9
c^2 = 5
c = √5