I'm pretty sure I put 0.7 as the approx. to one decimal place. I got g(0.7) > 0 and g(0.75) < 0 so the root is between 0.75 and 0.7. So its 0.7 to one dec place?
For Question 3dii to get the shortest chord, x needs to be exactly half of L. So L=2x. You just sub this into the equation:
x^2 - Lx + 12 = 0
Becomes
x^2 - 2x^2 +12 = 0
-x^2 +12 = 0
x^2 = 12
Thus x is the positive square root of 12, since a length can't be negative.
If you can't or don't want to leave the exam just yet, you can always try a few things from the website below. Some of the suggestions sound...interesting ;)
http://web.mit.edu/voodoo/www/is744/exam.html