1. Start by putting x = 0 in the equation this gives y = c at x = 0. This will eliminate some of the choices. 2 . Now put x = infinity. => y = a x (infinity)^2. This means only those graphs which go to infinity for large values of x can be the correct one. Note here that if a is negative the curves go to - infinity for infinite x. 3. Another trick is to look at the graphs and see where the curves intersect the x - axis i.e. The roots of the equation or the values of x for which y = 0. Put these values in the given expression to see if these satisfy it. 4. Remember, these graphs will always be parabolas. So you can chuck any circles, ellipses etc.