1) 2a+c=-10

Question

b-c=15
a-2b+c=-5

2) x+y+z=3
13x+2x=2
-x-5z=-5

3)x-y+3z=3
-2x+2y-6z=6
y-5z=-3

4)3x-2y+z=1
-x+y-z=2
5x+2y+10z=-39

5)3x-2y+z=1
-x-y-z=-3
3x-2y+3z=-12

Answer 1

http://www.wolframalpha.com/input/?i=2a%2Bc%3D-10%2C+b-c%3D15%2C+a-2b%2Bc%3D-5+

2a+c=-10, b-c=15, a-2b+c=-5