X^2-5x, what number must be added to complete the square

Thank you :)

add the square of half of the -5, which is 25/4

You have to add 6.25. You halve the coefficient on the x^1 term, and square that. So the expression is x^2 - 5x + 6.25, which is (x - 2.5)^2.

(5/2)^2

b^2=4ac
a=1,b=-5,c=?
(-5)^2=4*1*c
25=4c
c=4/25