Solve system of equations using substitution:
1) 2x - y = 16
2y - x = -8

assign two variables and equations. solve.

jack and cameron are playing a game of paper football. by their rules, you can score a 5-point touchdown or a 7-point touchdown. in the game, there have been 13 touchdowns scored for a total of 71 points. how many of these touchdowns were 7-point touchdowns?

1)
2x-y=16
-2y-x=-8
You need to solve on of the equations for x or y.  I will solve the first equation for y.
2x-y=16
-y=-2x+16
y=+2x-16
Then you can substitute that y value for the y value in the second equation (2y-x=-8) can be written as 2(2x-16)-x=-8 since y=2x-16.
then you solve for x. 
4x-32-x=-8
3x=24
x=8
therefore y=0 since 2(0)-8=-8
I will complete the second part of the question either in the comments or a new answer later when I have more time.