Answer
$(\frac{b}{2})^{2}$
Work Step by Step
To find the term required to create a perfect square trinomial, take the coefficient of the $x$-term, divide it by two, and then square it. (This only works if the value of $a$ is $1$).
$x^{2}+bx=c$
the coefficient of the $x$-term..... $b$
divide it by 2..... $(\frac{b}{2})$
and square it...... $(\frac{b}{2})^{2}$
