How do I solve a cubic?

You can solve a cubic by applying factor theorem. This is where you plug different values for x into your cubic f(x) = ax^3 + bx^2 + cx + d such that x is a factor of d, until you find a value t for which f(t) = 0. From here factorise the cubic and solve the remaining quadratic equation by the same method, by factorisation, completing the square or by the quadratic formula.