Given that $x+y=10$ and $2x+y=13$, what is the value of $x^2-y^2$?
x + y = 10 ---> -x - y = -10
2x + y = 13 ---> 2x + y = 13
Add down the columns: x = 3
Substituting into the first equation: y = 7
x2 - y2 = (3)2 - (7)2 = 9 - 49 = -40
