4 x - y = 7 Add y to both sides
4 x - y + y = 7 + y
4 x = y + 7 Subtract 7 to both sides
4 x - 7 = y + 7 - 7
4 x - 7 = y
y = 4 x - 7
5 x + y = 20
5 x + 4 x - 7 = 20
9 x - 7 = 20 Add 7 to both sides
9 x - 7 + 7 = 20 + 7
9 x = 27 Divide both sides by 9
9 x / 9 = 27 / 9
x = 3
y = 4 x - 7
y = 4 * 3 - 7
y = 12 - 7
y = 5
4x-y=7
5x+y=20
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