The formula for the total resistance $R_T$ of two resistors $R_1$ and $R_2$ connected in parallel is given by $\frac{1}{R_T}=\frac{1}{R_1}+\frac{1}{R_2}$. If $R_1$ is a fixed resistance of 6 ohms, and the equation is rewritten to express $R_2$ in terms of $R_T$, in the form $R_2=\frac{a{R}_T}{R_T-b}$, where $a$ and $b$ are constants, what is the value of $a$?