### Home > CCA2 > Chapter C > Lesson C.1.1 > ProblemC-18

C-18.

Where do the graphs below intersect?
You should be able to do these without a graphing calculator.

1. $2x + y = 10$
$x + y = 25$

1. $2x + y = 10$
$x^2 + y^2 = 25$

Multiply the second equation by $-1$ and add the two equations together.

$2x + y = 10$
$-x - y = -25$

$x = -15$

Use the $x$-value to calculate the $y$.

$y - 15 = 25$

$y = 40$

$(-15, 40)$

Solve the first equation for $y$ and substitute it into the second.