Can you explain number 7 on P. 72 of PWN the SAT 4th edition? Thank you.

Yep! Use elimination again here. Recognize that if you multiply the first equation by 5, you’ll have the easy opportunity to eliminate the *x*-terms.

From there, you know that *y* = 1, so you know the answer must be A because no other choice has 1 for its *y*-coordinate. Might as well keep solving for *x* to be sure, though…