<PRE><B>Question 436402</B>
m-7n=56  m=7n+56
7m+n=42
7(7n+56)+n=42
49n+392+n=42
50n=-350
n=-7
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7m-7=42
7m=49
m=7
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proof
m-7n=56
7-7*-7=56
7+49=56</PRE>