According to the thyoreme of Pythagor, the side of a traingle squared plus the other side of the triangle squared is equal to the hypothenuse squared. The hypothenuse is the longer side of a triangle and in this case our diagonal. Then we find the square root of 15(2) + 11(2). It is 18.60349683.
That long number is equal to 18.6
<u><em>PLEASE MARK BRAINLIEST IF IT HELPED</em></u>