Question 76596
(x-3)(x-3)=x^2-81
x^2-6x+9=x^2-81 the x^2 factors cancel out thus:
-6x=-81-9
-6x=-90
x=-90/-6
x=15 cm. the original length of the sides of the square.
proof
(15-3)(15-3)=12^2-81 
12*12=15^2-81
144=225-81
144=144