Basic formula of Binomials.

i.e $(a+b)^2=a^2+2ab+b^2$

The question is given in the form of left hand side of the above formula(i.e $(x+7)^2$)

Where a=x,b=7

$(x+7)^2=x^2+2\cdot x\cdot7+7^2$ $=x^2+14x+49$ | Solving the problem and assigning a,b value in the above formula |

$(x+7)^2=x^2+14x+49$ | Using the above formula gets you the result |