Apark is shaped like a rectangle with a length 3 times its width w. what is a symplified expression for the distance between opposite corners of the parks

Let the width of the park be; w

width=w
length=3w
 
and let the distance between opposite corners of the park be; d

then the expression is; (w)squared +(3w)squared = (d) squared

When i did the math i got 11 cm

Ais the correct answer