Descartes' Rule Of Sign Is Not Enough!

Algebra Level 4

$\large { x }^{ 6 }-12{ x }^{ 5 }+a{ x }^{ 4 }+b{ x }^{ 3 }+c{ x }^{ 2 }+dx+64 =0$

Let $a,b,c,d$ be all constants.

If all of the roots of the above equation are positive, find $b + c - a$ .