If a triangle has side lengths of 9 cm, 12 cm, and 15 cm, what type of a triangle is it?

Let "a", "b" and "с"  be sides of the triangle ("с" is the longest side).
The triangle will be:

right if       a² + b² = c²
аcute if     a² + b² > c²    
obtuse if   a² + b² < c²    

We have a=9, b=12 and c=15

9² + 12² = 81 + 144 = 225
and
15² = 225

225 = 225   ⇒  right triangle.

What the heck don’t understand