Could you help me on number 9 on page 70 please? I kept getting a 4.312 answer

Sure. Here’s the problem:

The first thing you should do here is get rid of that infernal \sqrt{5}:

    \begin{align*}2x\sqrt{5}-3y\sqrt{45}&=12\sqrt{5}\\2x\sqrt{5}-3y\sqrt{9}\sqrt{5}&=12\sqrt{5}\\2x\sqrt{5}-3y(3)\sqrt{5}&=12\sqrt{5}\\2x\sqrt{5}-9y\sqrt{5}&=12\sqrt{5}\\2x-9y&=12\end{align*}

From there, you can subtract to eliminate the 9y terms:

2x – 9y = 12
– (5x – 9y = 18)
–3x = –6
= 2

Now use x = 2 to find y:

5(2) – 9y = 18
10 – 9y = 18
–9y = 8
y = –8/9

The question asks for the sum of x and y, so add them up!

2+\left(-\dfrac{8}{9}\right)=\dfrac{10}{9}