Use the formula (a+b+c)2=a2+b2+c2+2ab+2bc+2ca By substitute a=3k,b=−4r,c=−2m, we get Thus, (3k−4r−2m)2=(3k)2+(−4r)2+(−2m)2+2(3k)(−4r)+2(−4r)(−2m)+2(−2m)(3k) ⇒(3k−4r−2m)2=9k2+16r2+4m2−24rk+16rm−12km .......(1) Similar way, (3k+4r−2m)2=9k2+16r2+4m2+24rk−16rm−12km ......(2)
Adding equation(1) and (2), we get the required expression as (3k−4r−2m)2+(3k+4r−2m)2=9k2+16r2+4m2−24rk+16rm−12km+9k2+16r2+4m2+24rk−16rm−12km =18k2+32r2+8m2−24mk