How do you simplify 5x+4+9x?

Since 5 and 9 have same variable x, you can add these two. But since 4 has no variable and is different from a term containing x, you cannot add it with other terms.
Therefore,
${\displaystyle =5x+4+9x}$
${\displaystyle =14x+4}$
You can even take a digit common. In this case 2 goes common for both terms so
Answer: ${\displaystyle 2(7x+2)}$