2y is in the first equation so if you change 0.5x+y=17 to y=17-0.5x and then double it to 2y=34-x, you can  substitute and find x. And then you can find out y.
How to find x:
0.5x+y=17
y=17-0.5x
2y=34-x
3x-2y=8
3x-34-x=8
x=21
How to find y (with previous info.)
x=21
3x-2y=8
3*21-2y=8
63-8=2y
55=2y
27.5=y
