Quick question on tables and integration

ok so here is the problem

$$\int\frac{dx}{\sqrt(x^2-4x)}$$

the table integral I am supposed to use is this
$$\int\frac{du}{\sqrt{u^2-a^2}}=ln(u+\sqrt{u^2-a^2}+C$$

Is it proper to make my u=x^2 and a=2x^(1/2)
I am asking because the solution guide tells me to complete the square, and then proceed to pick the u and a

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No. a is assumed to be constant.

so if a is any real number i can do this, but if any nonconstant variable is there than I need to find another way? Sorry it has been a while since Ive done this.

dextercioby