$$\begin{align} u(x_1x_2) & = -a\ln{a\over (a+b)x_1} -b\ln {b\over (a+b)x_2} \\ & = a\ln x_1 + b\ln x_2 + \text{constant}. \end{align}$$
Could somebody please explain how this is simplified? I'm confused with the steps needed to give us these signs.