Draw a rectangle inside a circle with its corners touching the circle. Let width =2w and height=2h. Draw a radius from centre to one corner Complete to make a right angled triangle by drawing a line from centre perpendicular to the height. This triangle has sides w,h and 14. So h^2+w^2=196

Strength is S=k(2w)(2h)^2=8kwh^2

Replace h^2 by 196-w^2

Find dS/dw and solve dS/dw=0