Answer:
n= -3
Step-by-step explanation:
[tex]3n+9n=-2(7-4n)-(1+n)\\\\\mathrm{Add\:similar\:elements:}\:3n+9n=12n\\\\12n=-2\left(7-4n\right)-\left(1+n\right)\\\\\mathrm{Expand\:}-2\left(7-4n\right)-\left(1+n\right):\quad 7n-15\\\\12n=7n-15\\\\\mathrm{Subtract\:}7n\mathrm{\:from\:both\:sides}\\\\12n-7n=7n-15-7n\\\\Simplify\\\\5n=-15\\\\\mathrm{Divide\:both\:sides\:by\:}5\\\\\frac{5n}{5}=\frac{-15}{5}\\\\Simplify\\\\n=-3[/tex]
