express x^2 - 5x + 1/4 in the form (x-a)^2 - b
"a" and "b" are both constants,
they have no 'x'.
When you multiply out $\displaystyle (x-a)^2$ you get
$\displaystyle (x-a)(x-a)=x(x-a)-a(x-a)=x^2-ax-ax+a^2=x^2-2ax+a^2$
This means $\displaystyle -5=-2a$
So you've got the value of 'a' now.
Use this value of 'a' to find 'b'.