I did this more by trial and error than by 'real' maths.
ax^2+c>=0
x^2 is always greater or equal to 0
If a is positive then ax^2 is 0 or positive
If c is zero or positive then the answer has to be greater or equal to 0.
If there is only one correct answer then it has to be
a>0 and c>=0
+118677 
