9x^2 + 25 - 30x can be expressed as the square of = ?

Answer: Option D

$$\begin{array}{rl}& \text{We have to find}\\ & =\sqrt{9x^2+25-30x}\end{array}$$
 $$=\sqrt{{\left(3x\right)}^2-2.3x.5+{\left(-5\right)}^2}$$   $$\left\{\because a^2-2ab+b^2={\left(a+b\right)}^2\right\}$$
$$\begin{array}{rl}& =\sqrt{{\left(3x-5\right)}^2}\\ & =3x-5\end{array}$$