I dont understand how to solve for a variable when there are two of the same variable, how do i solve for R1?

[tex] \bf R_1R_2=RR2+RR_1\implies R_1R_2-RR_1=RR2
\\\\\\
\stackrel{\textit{common factor}}{R_1(R_2-R)=RR_2}\implies
R_1=\cfrac{RR_2}{R_2-R} [/tex]