$\frac{UA}{F_{A0}}(T_a-T)-(x_2-x_1)H_{Rx}=(C_{PA}+C_{PB})(T-T_0)$

Mole balance for the second reactor,

$V_2=\frac{F_{A0}(x_2-x_1)}{-r_{A2}}=\frac{F_{A0}(x_2-x_1)}{k{C}_{A0}^2(1-x_2{)}^2}$

Mole balance for reactor $1$ :

$V_1=\frac{F_{A0}{x}_1}{k{C}_{A0}^2(1-x_1{)}^2}$

$k=\frac{F_{A0}x}{V{C}_{A0}^2(1-x{)}^2}=\frac{vx}{V{C}_{A0}(1-x{)}^2}$

write $itin$ term $ofx2$