Answer:
2nd choice
Step-by-step explanation:

We're dividing fractions, so we will flip the one in the denominator upside down and we will multiply it with the numerator.

Let's equal this too 8/x

Cross multiply.

Divide both sides by 15.
The factoring can be done similarly to a quadratic equation thanks to x^4 being the square value of x^2.
<span>x^4 + 6x^2 - 7
x^4</span><span> - x^2</span> + 7x^2 - 7
(x^4 - x^2) + (<span>7x^2 - 7)
</span>x^2(x^2 - 1) + 7(<span>x^2 - 1)
</span>(x^2 + 7)(x^2 - 1)
<span>(x^2 + 7)(x - 1)(x + 1)
</span>
Factored completely we get: <span>(x^2 + 7)(x - 1)(x + 1)</span>
Number 21 would be 6 39/40