you want to find the area of the whole square first and then calculate the area of the of the white triangle. (Area= length x width)
area= 18x18
area of whole square= 324
now you want to find the area of the white triangle
you know the base of that triangle is 18 m because the square is the same length on all of the sides.
area of triangle= b x h/2
area of triangle = 18x9/2
area of triangle= 162/2
area of triangle = 81
now subtract the area of the whole square and the area of the triangle
area of blue part= 324-81
area of blue part= 243
and
area of white part (which is the triangle)
=81