considering an equation ax + by + c =0, im supposed to give examples of a, b, c for which the equation does not represent a straight line
cant find any
ax+by+c=0
then
a=-(by+c)/x
now if y,c,b,x are rational numbers then a will also be a rational number (case of 0 has already been discussed)
so i don't think there is any value of a,b,c (except a=b=c=0) for which equation ax+by+c=0 do not represents a line.