Answer
$y=\pm \sqrt {20-\dfrac {4}{5}x^{2}}$
Work Step by Step
$\dfrac {x^{2}}{25}+\dfrac {y^{2}}{20}=1\Rightarrow \dfrac {y^{2}}{20}=1-\dfrac {x^{2}}{25}\Rightarrow y^{2}=20-\dfrac {4}{5}x^{2}\Rightarrow y=\pm \sqrt {20-\dfrac {4}{5}x^{2}}$
Now we can graph this equation using graphing calculator as asked in question