ok
assuming ya mean [tex]x^2-7x[/tex]
since the quadratic coefient is 1, take 1/2 of the liner coeficient and square it
the linear coefient is -7
half of that is -7/2
squared we get 49/4
add that to get
[tex]x^2-7x+\frac{49}{4}[/tex]
it factors to
[tex](x-\frac{7}{2})(x-\frac{7}{2})[/tex] or [tex](x-\frac{7}{2})^2[/tex]
you must add [tex]\frac{49}{4}[/tex] to make it a perfect square
