SEAMO 2020 — Paper E — Grade 9–10 Question 11

The monic cubic $p(x)=x^3+ax^2+bx+c$ has real roots $x_1,x_2,x_3$. Given $a+c=10$, $b=5$, find $(x_1^2-1)(x_2^2-1)(x_3^2-1)$.