Noun and の adjective
さんかくけい
さんかっけい
どうすれば、三角形のオニギリをにぎれるんですか？
How do you make onigiri into a triangle?
夏の夜空には、天の川をはさんでことざとわしざと白鳥ざの一とう星をむすんだ大三角形が広がります。
A big triangle, consisting of three magnitude stars from Lyra, Aquila, and Cygnus, stretches out in the summer night sky.
そこには、三角形の公園があります。
There is a triangle park there.
You already learned that 三角 means "triangle." Well this means triangle too, but it's the mathematical term. You should use it in math class.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.