The diagonal is equal to the hypotenuse of a right triangle having sides of length x and y. Here we are told that the width is 6 meters and the hypotenuse is 9 meters. So by the Pythagorean Theorem:
h^2=x^2+y^2 so we have:
6^2+L^2=9^2
36+L^2=81
L^2=81-36
L^2=45
L=√45
L=√(3*3*5)
L=3√5 meters