I was wondering if anyone could help? I was looking over some notes about completing the square, and can anyone tell me why this is true?

x^2+5x+3 =

(x+5/2)^2-(5/2)^2+3

I get the first part, (x+5/2)^2 because this makes up a complete square, but why is there a constant that is taken away?

Thanks alot