Answer
$$(6x^2+5)(7x^2-8) .$$
Work Step by Step
For the polynomial
$$ 42x^4 -13x^2 -40 $$
we check the factors of $ac=42(-40)=-1680$. The only factors that can give the right middle term are $-48$ and $35$, as follows
$$ 42x^4 -13x^2 -40 =(6x^2+5)(7x^2-8) .$$
