Sum of powers

Algebra Level 3

$\begin{cases} a+b+c=1\\ { a }^{ 2 }+{ b }^{ 2 }+{ c }^{ 2 }=21\\ { a }^{ 3 }+{ b }^{ 3 }+{ c }^{ 3 }=55 \end{cases}$

Given that $a,b$ and $c$ satisfy the system of equations above, find $(a-1)(b-1)(c-1)$ .