Given the equation of the circle:

if we associate the terms with the variable x together, we have:

now, we can complete the square of the expression x^2-6x by dividing 6 by 2, and calculate the square of the result. In this case, we have:

then, we have to add 9 on both sides of the equation to get the following:

notice that the center of the circle is located at the point (h,k) = (3,0). Therefore, the x coordinate of the center of the circle is x = 3