Use Cardano Vieta relations.
Certainly the equation can be easily solved using Ruffini's rule. However the sense of the problem is surely to use the fact that the roots are in arithmetic progression. An this, in order to see if the student knows the relations between roots an coefficients. In this case the relations:
lead to and to the biquadratic equation:
Of course it is not necessary to write the difference of the progression as , but it is correct.