Answer
The triangle is not a right triangle.
Work Step by Step
A right triangle has three sides that satisfy the equation
$c^2=a^2+b^2$
where $c$ is the longest side (hypotenuse) and $a$ and $b$ are the two other sides.
Substitute the given lengths of sides to the equation above to obtain:
$c^2=a^2+b^2
\\6^2=4^2+5^2
\\36=16+25
\\36\ne 41$
The triangle's side lengths do not satisfy the equation; therefore the triangle is NOT a right triangle.
