Using the property of the addition of logs:
[tex]\begin{gathered} \log _z(xy)=\log _z(x)+\log _z(y) \\ so\colon \\ \ln (10x)+\ln (C)=\ln (10x\cdot C)=\ln (10Cx) \end{gathered}[/tex][tex]\ln (y)=\ln (10Cx)[/tex]Take the exponential function of both sides:
[tex]\begin{gathered} e^{\ln (y)}=e^{\ln (10Cx)} \\ y=10Cx \end{gathered}[/tex]