Answer
$(x+7-7a)(x+7+7a)$
Work Step by Step
The product of a binomial sum and a binomial difference is: $(A+B)(A-B)=A^2-B^2$.
The square of a binomial sum is: $(A+B)^2=A^2+2AB+B^2$.
The square of a binomial difference is: $(A-B)^2=A^2-2AB+B^2$.
Hence here: $x^2-49a^2+14x+49=\\x^2+2\cdot x\cdot7+7^2-(7a)^2\\=(x+7)^2-(7a)^2\\=(x+7-7a)(x+7+7a)$
