Step 1 :

After factoring out $ -1 $ we have:

$$ -4x^{2}+x+16 = - ~ ( 4x^{2}-x-16 ) $$

Step 2 :

Step 2: Identify constants $ a $ , $ b $ and $ c $.
$ a $ is a number in front of the $ x^2 $ term $ b $ is a number in front of the $ x $ term and $ c $ is a constant. In this case:

$$ a = 4 , b = -1 ~ \text{ and } ~ c = -16 $$

Step 3: Multiply the leading coefficient $\color{blue}{ a = 4 }$ by the constant term $\color{blue}{c = -16} $.

$$ a \cdot c = -64 $$

Step 4: Find out two numbers that multiply to $ a \cdot c = -64 $ and add to $ b = -1 $.

Step 5: All pairs of numbers with a product of $ -64 $ are:

PRODUCT = -64
-1     64	1     -64
-2     32	2     -32
-4     16	4     -16
-8     8	8     -8

Step 6: Find out which factor pair sums up to $\color{blue}{ b = -1 }$

Step 7: Because none of these pairs will give us a sum of $ \color{blue}{ -1 }$, we conclude the polynomial cannot be factored.

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