If a² + b² = c², then the triangle is right.
So we have (8)² + (11)² ? (13)²
(8)² is 64, (11)² is 121, and (13)² is 169.
So we have 64 + 121 ? 169
64 + 121 is 185 and we can see that 185 > 169.
This triangle would not be a right triangle.
In fact, it would be an acute triangle.
So no, it's not a right triangle.