The given expression is 3x2 - x - 4. We need to factor this quadratic expression.

We need to factor this quadratic expression.
The student has correctly split the middle term $-x$ into $-4x + 3x$.

$$3x^2 - x - 4 = 3x^2 - 4x + 3x - 4$$

From the first two terms, $3x^2 - 4x$, the common factor is $x$.
From the last two terms, $3x - 4$, the common factor is 1.

$$3x^2 - 4x + 3x - 4 = x(3x - 4) + 1(3x - 4)$$

$$x(3x - 4) + 1(3x - 4) = (3x - 4)(x + 1)$$

The factored form of the expression is:

$$(3x - 4)(x + 1)$$