How ?

first split the product of the constant term and the coefficient of x squared in big factors. If the factors that you get don't add up to the coeficient of the middle term then go for the prime factors. In this case -1*(-306)=306=9*34(this does not work out, so adjust these factors again) =18*17(now, this does work, so split the middle term as 35x=18x+17x) If product is positive it shows that both numbers must be of the same sign, since the middle term coef is +, so both the numbers have to be positive.)