Use the elimination method.

{-2x + 3y = 20

{4x + 4y = -15

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Solve `{[-2x+3y=20],[4x+4y=-15]}` :

Choose to eliminate x; the coefficients are opposite in sign and 4 is a multiple of 2.

Multiply the first equation through by 2 and then add to the second equation:

-4x+6y=40

4x+4y=-15

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10y=25

`y=5/2` Substitute for y in the second equation (this eliminates the fraction) to get:

`4x+4(5/2)=-15 ==>4x+10=-15 ==>4x=-25==>x=-25/4`

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The solution is `(-25/4,5/2)`

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The graph of the two lines:

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