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:

That tells you that , 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.