Question 551827
Hint: If the side length of the square is x, then {{{2x^2=d^2}}} where d is the diagonal. So because the diagonal is 18 m, we know that d = 18 and {{{2x^2=18^2}}}



Once you have the side length, you can plug it into the formulas



Area: {{{A=s^2}}} 



Perimeter: {{{P=4s}}}



where 's' is the side length