[ATTACH=full]328[/ATTACH]
sqrt(n^2-12^2)=3^2...........square both sides (sqrt(n^2-12^2))^2=(3^2)^2 n^2-144=(9)^2 n^2=81+144 n^2=225 n=sqrt(225) n=15 check: sqrt(15^2-12^2)=3^2 sqrt(225-144)=3^2 sqrt(81)=3^2 9=3^2 3^2=3^2