Question 301316
Set-Up:
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Equation 1: {{{5X - 2Y = -7}}}
Equation 2: {{{8 = Y - 4X}}}
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Solution:
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You need to solve one of the equations for one variable.  in this case I thin that solving equation 2 for Y is the easiest.
Equation 2: {{{8 = Y - 4X}}} Add 4X to both sides
{{{8 + 4X = Y}}}
Now you can plug (8 + 4X) into equation 1 for Y
Equation 1: {{{5X - 2Y = -7}}}
{{{5X - 2*(8 + 4X) = -7}}} Simplify
{{{5X - 16 - 8X = -7}}} Simplify
{{{-3X - 16 = -7}}} Add 16 to both sides
{{{-3X = 9}}} Divide both sides by -3
{{{highlight(X = -3)}}}
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Now plug -3 into equation 2 for X
{{{8 + 4X = Y}}}
{{{8 + 4*(-3) = Y}}}
{{{8 - 12 = Y}}}
{{{highlight_green(-4 = Y)}}}