Answer :
Given,
The equation is, x^2-12x=28.
Comapring the given equation with
[tex](a-b)^2=a^2-2ab+b^2[/tex]On comapring it obtains that,
[tex]\begin{gathered} a^2=x^2 \\ a=x\ldots\text{.}(1) \end{gathered}[/tex]Similarly,
[tex]\begin{gathered} -2ab=-12x \\ -2\times x\times b=-12x \\ b=\frac{-12x}{-2x} \\ b=6 \end{gathered}[/tex]Now,
[tex]b^2=6\times6=36[/tex]Hence, 36 is added to both sides to make the equation complete square.
Hence, option d is correct