$\overrightarrow{ r }=(2 \hat{ i })-(2 \hat{ i }+3 \hat{ j }+4 \hat{ k })=-3 \hat{ j }-4 \hat{ k }$
$ \overrightarrow{ F }=4 \hat{ i }+3 \hat{ j }+4 \hat{ k }$
$\vec{\tau}=\overrightarrow{ r } \times \overrightarrow{ F }=\left|\begin{array}{ccc}\hat{i} & \hat{ j } & \hat{ k } \\ 0 & -3 & -4 \\ 4 & 3 & 4\end{array}\right|$
$\quad=\hat{ i }(-12+12)-\hat{ j }(0+16)+\hat{ k }(0+12)$
$=-16 \hat{ i }+12 \hat{ k }$
$\therefore \quad|\vec{\tau}|=\sqrt{16^{2}+12^{2}}=20$
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