Can you explain number 6 on P. 72 of PWN the SAT 4th edition? Thank you

Sure. For this question, you should see right away that the system lends itself to elimination of the y-terms. Subtract the second equation from the first:

    \begin{align*}3x-2y&=16\\-\left(\dfrac{2}{5}x-2y\right)&=-4\\\rule{2cm}{0.4pt}&\rule{1cm}{0.4pt}\\\dfrac{13}{5}x&=12\end{align*}

That tells you that x=\dfrac{60}{13}, which is enough to eliminate every choice but D.

You can also solve this by using your graphing calculator, as long as you get everything in = form first:

Now check the answer choices to see which line up with those decimals. Sure enough, choice D does.