Consider as the beginning of a complete square where the final square number is missing. If you have then the final square is . In your case a = 1. Thus the final number is .
If you add 9 to get a complete square you must subtract the 9 at once so that the equation is still true:
Now collect the first 3 summands:
The equation belongs to a parabola opening up. Thus the vertex is the minimum point. At x = 3 the function has ist smallest value f(3) = -7
And now it's your turn!