X² + y² + z² -xy - xz - yz = 0
multiplying 2 on both sides
2x² + 2y² + 2z² - 2xy - 2xz - 2yz = 0
x² + y² - 2xy+ x² + z² - 2xz + z² + y² - 2yz = 0
(x - y)² + (x - z)² + (y - z)² = 0
so the value are in square so they can't be negative so they must be indivudually zero to satisfy the equation
so x - y = 0 ⇒ x = y
x - z = 0 ⇒ x = z
y - z = 0 ⇒ y = z
so hence x = y = z
Therefore the triangle is an equilateral triangle since sides are equal