Answer:
See below
Explanation:
When nether of the variable will fall out when adding together. For example:
2x + 5y = 13
-2x + 7y = 17
When I add these equation together, the variable x will drop out because 2x - 2x = 0 and we would be left with other the y variable which we then can solve for. 12y = 30
Now another example where the neither the x or y falls out.
x + 5y = 12
2x + 3y= 9
When I add these together, neither variable falls out.
3x + 8y = 21
BUT, if I multiply x + 5y = 12 though by -2 I get
-2x -10y = -24
Now when I add the two equation together the x's will drop out.
-2x -10y = -24
2x + 3y = 9
-7y = --15