cubic equation is like (x-a)(x-b)(x-c)=0
from here we will get a general cubic equation like x3 - (a+b+c)x2 +(ab+bc+ca)x - abc=0
so from this you can say
a+b+c=4
ab+bc+ca= -11
abc=-30
Now from these three equation if roots are not that much complex , we can easily guess from guessing factors of 30 that will make sum=4 , and this time we're lucky and can guess like a=2 b=5 c=-3
another method is , if possible try to make factor of given equation
like 3x3-21x2+5x-35 = 3x2(x-7)+5(x-7)...
overall there is no standard way to solve cubic equations , we have to play with little math.
