ok from my math knowledge here is what I believe is the answer for each of these:
1) elimination: because you can just multiply the bottom equation (both sides of it!) by 2, and so eliminate the y variable. and from there it's easy to solve for x, and then you can use the answer for x to find y by substitution into one of the equations.
2) substitution: because you can see that it's easy to solve for either x OR y. and once you have the equation for either x= something or y= something you can just substitute that into one of the equations.
