Is a triangle with the side lenths of 4mm, 11mm, 15mm a right triangle

The triangle with side lengths 4,11,15 does not even exist.

According to the Triangle Inequality Theorem, in order for the triangle to even exist, the sum of the two shorter lengths must be strictly greater than the length of the third side.

4 + 11 = 15.

15 is not greater than 15.