Precalculus (6th Edition) Blitzer

$\frac{x^2}{25}+\frac{y^2}{9}=1$
Step 1. From the given conditions, we have $c=4, a=5$; thus $b=\sqrt {5^2-4^2}=3$ Step 2. We can write the equation as $\frac{x^2}{25}+\frac{y^2}{9}=1$