3x + y= 17 (a)
4x + 2 y = 20 (b)
Multiply (a) by 2:
6x + 2y = 34 (a)
4x + 2y = 20 (b)
make: (a) - (b)
2x = 14 -----> x =7
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Replace the value of x = 7 in one of anterior equation:
3x + y= 17
3 x 7 + y = 17
21 + y = 17
y = 17 - 21
y = -4