Idon't understand this, can somebody explain it to me?

Looking at the equation, you could see that it is in vertex form. Vertex form looks like this: y=a(x-h)^2+k where a is the slope to find the first point away from the origin and (h,k) is the vertex. So, the vertex is (3,-1). The first point away from the origin is up two, over one, so the slope is 2. This means that a is 2. One way you could check this is by putting 2 in for a. If you put it into standard form, it would be 2x^2-12x+17. To find the x in the vertex when the equation is in standard form we use -b/2a. So it would give us 3 for x. If we put in 3 for x, we get -1 for y, so the vertex is (3,-1). This means that a is 2. Sorry for this lengthy answer, hope it helps.