Ich habe die Gelegenheit genutzt und ihn angerufen.
I took the opportunity and called him.
Wir werden eine Gelegenheit finden, uns zu treffen.
We will find an opportunity to meet.
Note
The article is part of the word: die Gelegenheit. That way you learn the gender with it.
Remember die Gelegenheit for good. ripiti brings the word back just before you forget it, with these sentences. Free, no account needed, works offline.