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.)