For elimination, you need to eliminate one of the variables by subtrating the two equations.
To do that, you need to multiply the equation 3x+y=17 by 2, which becomes 6x + 2y = 34.
Now you can subtract, but also multiply 6x+2y=34 by -1 so you can subtract:
4x + 2y = 20
-6x - 2y = -34
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-2x + 0 = -14
x = 7
Now that you know x=7, plug it back in to one of the original equations to solve for y.
4x + 2y = 20
4(7) + 2y = 20
28 + 2y = 20
2y = -8
y = -4
