The function f(x) is a quadratic function and we can define, for a quadratic function, its vertex form as follows:

Where a, h, and k are numbers
Notice that if we expand the previous expression, we get:

So, from the function f(x)=x^2-6x+13 we can see that a=1 and that a*h=6, i.e.,

Furthermore, notice that:

and h=6, a=1!

We have everything we need to write the vertex form:

Now, we can calculate its minimum value!
Looking at the function we can notice that its graph is a parabola that opens towards the +y direction (it has a 'U' shape). Therefore, there is a point that is the lowest point of the graph!
We can calculate it by means of the derivative:

And, to find the minimum we do f'(x)=0

And x=3 gives us the minimum value of f(x)! In this way:
