Let me say x1, x2, x3,x4 and x5 are the roots of P(x)=0
Now roots will be

Now If I divide P(x^12) by P(x) , remainder will be of deg=4
[this is a rule, deg of remainder is one less than deg of divisor]
So, P(x^12) = P(x)(quotient) + (ax^4 + bx^3 + cx^2 + dx^1 + e )
Now in above eqn there are 5 variables which can be found by puting x= x1, x2, x3,x4 and x5
and remember P(x1), P(x2), P(x3),P(x4) and P(x5) == 0
That’s it!!