The first thing to do is to set this equal to zero, then move the constant over to the other side of the equals sign so we only have the x terms to worry about. 
. Now we're ready to complete the square. We take half the linear term, square it, and add it in to both sides. Our linear term is -6x. Half of 6 is 3, and 3 squared is 9, so we add 9 to both sides: 
. Now we will do the math on the right to get 4, and on the left we will express the perfect square binomial we created while completing this process. 
. Now we can move the constant back over by subtraction to get 
. Now we can see that the vertex is (3, -4). Since this is a positive parabola, it opens upwards, like a cup, which means that the lowest part of the parabola, the vertex, is sitting at the bottom, or at a minimum value. So our answer is C