1. in the equation a*t^2+b*t+c=0, consider this case: a=0.

2. maybe you will get two positive value of t, and you should check both of them, because the dart will go through the plane twice.

I got AC after getting Crash & WA for the reasons above.

I considered this 2 cases and even the case: b=0, but anyway I got WA 6 again :(

t can be only positive? OMFG! So stupid mistake))) Now I've got 'AC' Thank you :D