The problem asks us to factor the linear expression $21 - 4x$. We are given four options: "Can't be factored", $4(81 - 16x)$, $4(5 - x)$, and $7(3 - 4x)$.

1. Problem Description

The problem asks us to factor the linear expression $21 - 4x$. We are given four options: "Can't be factored", $4(81 - 16x)$, $4(5 - x)$, and $7(3 - 4x)$.

2. Solution Steps

We are looking for a factored form of the expression $21 - 4x$. Let's examine each of the provided options:

* "Can't be factored": We should check if the expression can actually be factored before accepting this option.

* $4(81 - 16x) = 324 - 64x$. This is clearly not equal to $21 - 4x$.

* $4(5 - x) = 20 - 4x$. This is not equal to $21 - 4x$.

* $7(3 - 4x) = 7(3) - 7(4/7 x) = 21 - 28x$. This is not equal to $21 - 4x$. However, a mistake was made with the $7(3-4x)$ option in the options. Let's instead test $7(3 - (4/7)x) = 21 - 4x$

Now we consider factoring a constant term out of the expression $21 - 4x$. We notice that the coefficients 21 and -4 do not share any common factors other than

1. Thus, we cannot factor out any integer other than

1.

Therefore, the expression $21 - 4x$ cannot be factored further using integer coefficients.

3. Final Answer

Can't be factored