Which of the following equations is used to find the value of c (the distance to the foci) for an ellipse?

The equation of ellipse:
[tex]\dfrac{(x-h)^2}{a^2}+\dfrac{(y-k)^2}{b^2}=1[/tex]
The formula of c (the distance to the foci)
[tex]c^2=a^2-b^2\qquad|\text{add}\ b^2\ \text{to the both sides}\\\\c^2+b^2=a^2[/tex]