Question 948909
"...on the left side..."


Some of us can recognize the square trinomials with experience and simple understanding.  NOT A nor D, since those just appear to not be square trinomials on the left.  C is a square trinomial on the left.


You might wonder about B.
It is not.
Try factoring it.
(x______-3)(x______-3), will not work because the x term is -6x.
(x_____3)(x______3), will not work because the constant term is +9.