document.write( "Question 116555: Solve the system by substitution. 3x – y = –7, x + y = –9 \n" ); document.write( "
Algebra.Com's Answer #84725 by jgr45(31)\"\" \"About 
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Solve the system by substitution. 3x – y = –7, x + y = –9\r
\n" ); document.write( "\n" ); document.write( "To solve by substitution you need to isolate one of the variables by itself. Here either one works so I'll pick y.\r
\n" ); document.write( "\n" ); document.write( "y = -9 - x\r
\n" ); document.write( "\n" ); document.write( "Now, rewrite the first equation and solve it, replacing y with -9-x.\r
\n" ); document.write( "\n" ); document.write( "3x - (-9-x) = -7\r
\n" ); document.write( "\n" ); document.write( "3x + 9 + x = -7
\n" ); document.write( "4x + 9 = -7
\n" ); document.write( "4x = -16
\n" ); document.write( "x = -4
\n" ); document.write( "Plug -4 in for x:
\n" ); document.write( "y = -9 - (-4)
\n" ); document.write( "y = -5
\n" ); document.write( "So the point of intersection is (-4,-5).
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