Question 638983
substitution.
x + 3 = y
3x + 4y = 7 
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From the 1st equation we know that: y = (x+3)
Substitute (x+3) for y in the 2nd equation
3x + 4(x+3) = 7
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multiply what's inside the brackets
3x + 4x + 12 = 7
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Add the x's, subtract 12 from both sides
7x = 7 - 12
7x = -5
x = {{{-5/7}}}
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Find y
y = x + 3
substitute {{{-5/7}}} for x
y = {{{-5/7}}} + 3
Change 3 to 7ths
y = {{{-5/7}}} + {{{21/7}}}
y = {{{16/7}}}
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Solution: x = {{{-5/7}}}; y = {{{16/7}}}
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Check solution in the 2nd equation
3x + 4y = 7
Replace x and y
3({{{-5/7}}}) + 4({{{16/7}}}) = 7
{{{-15/7}}} + {{{64/7}}} = 7
{{{49/7}}} = 7
7 = 7; confirms our solutions,
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