Answer
The trinomial $x^{2}-30x+c$ is a perfect square when $c=15.$
Then $x^{2}-30x+225=(x-15)(x-15)=(x-15)^{2}$
Work Step by Step
$ x^{2}-30x+c\qquad$ ... find half the coefficient of $x$, which is $\displaystyle \frac{-30}{2}=-15$.
$\qquad$ ...square the result.
$(-15)^{2}=225\qquad$ ...substitute $c$ with $225$ in the original expression
$x^{2}-30x+225=(x-15)^{2}$
