I doubt this is the inequality you were given to solve. In any case, it has not solutions. Also, note thata/b<= 0 iffaandbhave opposite signs and b ≠ 0.

Use the fact that log(x) + log(y) = log(x * y).

Use sum-to-product identities.

Guess one root x₁ (an integer); use long division to divide the original polynomial by x - x₁ (the remainder is 0 by the polynomial remainder theorem). You get a quadratic equation that you can solve using the quadratic formula.