Solveeeeee itttt pleeeee

Dear student,
Coefficients of the 2nd, 3rd  and 4th terms in the given expansion are:
$$\begin{gathered} {}^{n}C_1, {}^{n}C_2 \mathrm{and} {}^{n}C_3 \\ \mathrm{We} \mathrm{have}: \\ 2\times {}^{n}C_2={}^{n}C_1+{}^{n}C_3 \\ \mathrm{Dividing} \mathrm{both} \mathrm{sides} \mathrm{by} {}^{n}C_2, \mathrm{we} \mathrm{get}: \\ 2=\frac{{}^{n}C_1}{{}^{n}C_2}+\frac{{}^{n}C_3}{{}^{n}C_2} \\ \Rightarrow 2=\frac{2}{n-1}+\frac{n-2}{3} \\ \Rightarrow 6n-6=6+n^2+2-3n \\ \Rightarrow n^2-9n+14=0 \\ \Rightarrow n=7 \left(\because n\ne 2 \mathrm{as} 2>3 \mathrm{in} \mathrm{the} 4\mathrm{th} \mathrm{term}\right) \end{gathered}$$
Regards