Question 551827
The diagonal length is the hypotenuse to a right triangle where the sides have equal length

From the Pythagorean theorem we know that 
{{{a^2+b^2=c^2}}}

Since the sides a and b are the same we can rewrite it as 
{{{s^2+s^2=18^2}}}
combine like terms
{{{2s^2=324}}}
divide both sides by 2
{{{s^2=162}}}
Since {{{s^2}}} is my area formula for a square, 162 is my area.
The perimeter will be 4*s
take the square root of both sides
s=12.73m
4s=50.91