[tex]\begin{gathered} u=10+3u \\ \text{Subtract u from both sides:} \\ u-u=10+3u-u \\ 0=10+2u \\ \text{Subract 10 from both sides:} \\ -10=10+2u-10 \\ -10=2u \\ \text{Divide both sides by 2:} \\ -\frac{10}{2}=\frac{2u}{2} \\ -5=u \\ u=-5 \end{gathered}[/tex]
