#x^2+(x-3)^2=225#
#"please note that "(x-y)^2=x^2-2xy+y^2#
#(x-3)^2=x^2-6x+9#
#x^2+(x^2-6x+9)=225#
#x^2+x^2-6x+9=225#
#2x^2-6x+9-225=0#
#2x^2-6x-216=0 " if we simplify , we obtain the equation below."#
#x^2-3x-108=0#
#"Now ,let us factor the equation above".#
#(x+9)(x-12)=0#
#"if "(x+9)=0" than "x=-9#
#"if "(x-12)=0" than "x=12#