The answer is graph (a)
Work Step by Step
We can deduce from the equation that we are dealing with a quadratic equation. It's quadratic since the highest power of x is two. A quadratic "parabola" ( just a fancy term for graph, don't let it scare you) has one of two shapes; it open's up either upwards or downwards. It DOESN'T look like a circle hence why we didn't choose (d). Furthermore the square ( raised to the power of two ) term has a negative co-efficient hence we know that the graph should be opening downwards. Only graph (a) satisfies the conditions hence it's the answer.