I'm retaking high school math so that I can go study something at some point but now I got stuck pretty early on in the material I'm studying from. Here's where I'm studying from, pretty close to the end of the page they pose the question:
In $(x-4)(x+k) = x^2+bx+c$ For which of the following values of $k$ will $b$ be positive? $-3;-1;0;4;5$
Now if you check on the website they say the answer is $k=5$ but I cannot figure out why. If someone can please explain how this sorta question works I'll be grateful because the website on which the question is asked doesn't explain how to do it.
Also please excuse the tags I used for this question as "basic-algebra" wasn't available and I don't know what abstract/linear algebra is but figured one of them must apply.