Hello,

If it's degree 4, then it can be written in this form :

Then, you have

This gives you 3 equations.

Since 3 is a zero with multiplicity of 2, this means that as well

Now, you only have 4 equations and you need a fifth ?

Since f is a polynomial with real coefficients, if a complex number is a zero, then its conjugate is also a zero.

Thus

Looks good to you ?