Solve {l}{quadP_{2}=frac{u^{2}}{R}=(frac{4R/3R+6)^{2}}{R}=16R/(3R+b)^2}{[2^4/3(6+R)]^2R+16R/(3R+6)^2}

$$\left. \begin{array} { l } { \quad P _ { 2 } = \frac { u ^ { 2 } } { R } = \frac { ( \frac { 4 R } { 3 R + 6 } ) ^ { 2 } } { R } = \frac { 16 R } { ( 3 R + b ) ^ { 2 } } } \\ { [ \frac { 2 ^ { 4 } } { 3 ( 6 + R ) } ] ^ { 2 } R + \frac { 16 R } { ( 3 R + 6 ) ^ { 2 } } } \end{array} \right.$$

Solve for P_2, u, R

$$P_{2}=\frac{u^{2}}{R}$$

$$u\in \mathrm{R}$$

$$R\in \mathrm{R}$$