Given that: x2+14x+49 =x2+7x+7x+49, (split middle term so that resultant is same)
=(x2+7x)+(7x+49), (group pair of terms)
=x(x+7)+7(x+7), (factor each binomials)
=(x+7)(x+7), (factor out common factor (x+7))
=(x+7)2
Aliter, use (a+b2)=a2+2ab+b2
⇒x2+14x+49=x2+2×7x+72=(x+7)2