If the sides of a triangle have lengths a, b, and c, and c^2=a^2+b^2, then the triangle is a right triangle.
This tells us that we can use the Pythagorean Theorem in reverse to test if a triangle is right. In general, the hypotenuse c has the greatest value. Let's substitute a=4, b=5, and c=6 into a^2+b^2=c^2, and see if they produce a true statement.
The values produce a false statement, so the described triangle is not a right triangle and, as a result, the given set of numbers is not a Pythagorean triple.