Help plz its math and i will give brainlest if u answer

not a right triangle

Step-by-step explanation:

We can use the Pythagorean theorem to see if it is a right triangle

a^2 + b^2 = c^2

15^2 + 12^2 = 21^2

225 + 144 = 441

369 = 441

This is not true so it is not a right triangle

Step-by-step explanation:

A triangle is a right angled triangle if sum of squares of two sides is equal to the square of third side.

12²+15²=144+225=369

21²=441

so a²+b²≠c²

it is not a right angled triangle.