You can use the distributive property twice. First, distribute (n+5) onto n, and then onto 4, like this:
color(white)=color(blue)((n+5))color(red)((n+4))
=color(blue)((n+5))color(red)n+color(blue)((n+5))color(red)4
=color(red)ncolor(blue)((n+5))+color(red)4color(blue)((n+5))
Now, use the distributive in each of these smaller parts:
color(white)=color(red)ncolor(blue)((n+5))+color(red)4color(blue)((n+5))
=color(red)ncolor(blue)n+color(red)ncolor(blue)5+color(red)4color(blue)((n+5))
=color(purple)(n^2)+color(blue)5color(red)n+color(red)4color(blue)((n+5))
=color(purple)(n^2)+color(blue)5color(red)n+color(red)4color(blue)n+color(red)4*color(blue)5
=color(purple)(n^2)+color(blue)5color(red)n+color(red)4color(blue)n+color(purple)20
Lastly, combine the like terms:
color(white)=color(purple)(n^2)+color(blue)5color(red)n+color(red)4color(blue)n+color(purple)20
=color(purple)(n^2)+color(purple)(9n)+color(purple)20
This is the result. (It is called a quadratic.)