Answer
not a right triangle.
Work Step by Step
For a right triangle with legs $a$, $b$ and the hypotenuse $c$, the Pythagorean theorem states that:
$$c^2=a^2+b^2.$$
We plug in the side lengths to check if the triangle is right:
$$
\sqrt{4^2+5^2}=\sqrt{41}\ne6
.$$
This statement is false, so the triangle is not right.
