Step 1: 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:

$$ a = 2 , b = 9 ~ \text{ and } ~ c = 4 $$

Step 2: Multiply the leading coefficient $\color{blue}{ a = 2 }$ by the constant term $\color{blue}{c = 4} $.

$$ a \cdot c = 8 $$

Step 3: Find out two numbers that multiply to $ a \cdot c = 8 $ and add to $ b = 9 $.

Step 4: All pairs of numbers with a product of $ 8 $ are:

PRODUCT = 8
1     8	-1     -8
2     4	-2     -4

Step 5: Find out which factor pair sums up to $\color{blue}{ b = 9 }$

PRODUCT = 8 and SUM = 9
1     8	-1     -8
2     4	-2     -4

Step 6: Replace middle term $ 9 x $ with $ 8x+x $:

$$ 2x^{2}+9x+4 = 2x^{2}+8x+x+4 $$

Step 7: Apply factoring by grouping. Factor $ 2x $ out of the first two terms and $ 1 $ out of the last two terms.

$$ 2x^{2}+8x+x+4 = 2x\left(x+4\right) + 1\left(x+4\right) = \left(2x+1\right) \left(x+4\right) $$

Polynomial Factoring Calculator