54 centimeters, 72 centimeters, and 90 centimeters. is it a right triangle?

We dont know yet, but we can check with pythagorean theorem.
Let's give names to all sides. The longest is possible hypotenuse =>
c=90, a=54 and b=72 => c^2 = a^2 + b^2 => 90^2 = 54^2+ 72^2 =>
8,100 = 2,916 + 5,184 =>  8,100 = 8,100 Identity proved that our triangle is right angled triangle.