Question 859689
-15x+7y=1 
3x-y=1
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In the second equation, isolate y by subtracting 3x from each side.
-y = 1-3x
Multiply each side by -1
y = 3x-1
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Substitute the known value of Y into the first equation.
-15x + 7(3x-1) = 1
Distribute
-15x +21x - 7 = 1
Add Xs
6x -7 = 1
Add 7 to each side
6x = 8
Divide 
x = 8/6
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If x = {{{8/6}}}, then y=3x-1, or {{{24/6}}}-1, or 4-1, or 3.
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Let's put the numbers back into the first equation to see if it works.
-15x+7y=1 
-15({{{8/6}}} + 7(3) = 1
-{{{120/6}}} + 21 = 1
-20 + 21 = 1
Success!