answer: $12
explanation:
we know that 2 CDs and 6 DVDs cost $156 and 1 CD and 9 DVDs cost $210.
this gives us the equations:
2x + 6y = 156
and
1x + 9y = 210
where x is CDs and y is DVDs
multiply the top equation by 9 and the bottom by -6 to solve for x:
18x + 54y = 1404
-6x - 54y = -1260
add from top to bottom:
12x = 144
divide both sides by 12 to get:
x= $12
