I explained a lot bc I’m not sure how in detail you want, so only pay attention to what you need :)
x^2 + 9x + 4
There is not way this could be factored further, because no two numbers have a product of 4 and a sum of 9. (No two numbers can be multiplied together to equal 4 and added together to give nine).
Because of this, the typical factoring technique that you would use for a polynomial in this form (ax^2 + bx + c) wouldn’t work:
Using the equation y= ax^2 + bx +c, you would typically multiply a with c. Then, you would try to find 2 numbers that can be multiplied together to equal a times c, but added together to equal b.
Using the equation I gave above, I’ll explain why this doesn’t work:
x^2 + 9x + 4
In this example, a times c is 1 times 4, which is 4. And b is 9. Therefore, you need 2 numbers that multiply to give 4 and add to give nine. But, the only numbers that multiply to give 4 are: 1,2,4.
1 + 4 = 5
2 + 2 = 4
Therefore, this polynomial is not able to be factored.