Can you explain number 9 on P. 72 of PWN the SAT 4th edition?

Sure. Recognize that \sqrt{45} simplifies to 3\sqrt{5}, so you can rewrite the first equation thusly: 2x\sqrt{5}-9y\sqrt{5}=12\sqrt{5}. Since there’s a \sqrt{5} in every term, you can just divide the whole equation by \sqrt{5} to eliminate the darn things! The equation simplifies to 2x-9y=12.

From there, this is a pretty easy problem. Solve for x by elimination:

    \begin{align*}2x-9y&=12\\-\left(5x-9y\right)&=18\\\rule{2cm}{0.4pt}&\rule{1cm}{0.4pt}\\-3x&=-6\\x&=2\end{align*}

From there, continue to solve for y:

    \begin{align*}5(2)-9y&=18\\10-9y&=18\\-9y&=8\\y&=-\dfrac{8}{9}\end{align*}

The question asks you for the sum of the coordinates of the ordered pair that satisfies the system of equations, so you have to add:

    \begin{align*}2+\left(-\dfrac{8}{9}\right)=\dfrac{10}{9}=1.\overline{1}\end{align*}

Gridding in either the fraction or the repeating decimal is fine as long as you fill all the boxes with the decimal (i.e., you enter “1.11”).