Answer:
The art is rectangular because 
(0.5) squared + (1.2) squared = (1.3) squared
Step-by-step explanation:
Given that the piece of art is rectangular in shape.
Length of piece of art = 0.5 meters
Width of piece of art = 1.2 meters
Kindly refer to the attached image in the answer area.
Two adjacent sides of a rectangle are given, the diagonal value can be found by using Pythagorean Theorem.
According to Pythagorean theorem:

Here, Hypotenuse will be the diagonal of the rectangle.
Base and Perpendicular will be the two adjacent sides.
Therefore,

Therefore, the answer is:
The art is rectangular because 
OR
(0.5) squared + (1.2) squared = (1.3) squared