An equation with degree 3

[mahtaban]

I have an equation similar:
8x^3+4cx^2+2(b-3)x+(a-2c)=0
a, b, c are real number.
if p, q , t be roots.
What can say about (p+q-2t) and (p^2+ q^2-2t^2)?
Please help me??
Thanks so much.

 

[Deveno]

hint: write your polynomial as 8(x-p)(x-q)(x-t) and multiply it out, then compare coefficients.

 

[mahtaban]

I know
p+q+t=c/2
pq+pt+qt=(b-3)/4
pqt=(2c-a)/8
but I need value for
(p+q-2t)??
(p^2+ q^2-2t^2)?
Please help me??