[tex]k=am+3mx\\\\k=m(a+3x)\\\\if\ \ \ a+3x \neq 0\ \ \ then\ \ \ m= \frac{k}{a+3x} [/tex]
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[tex]k=am+3m+3mx\\\\k=m(a+3+3x)\\\\if\ \ \ a+3+3x \neq 0\ \ \ then\ \ \ m= \frac{k}{a+3+3x} [/tex]