= Solution
Neglect atomic <binding energy> and take the target <electron> at rest. Use <natural units> temporarily. The final state contains three <electrons> and one <Positron>, so its threshold <invariant mass> is $4m_e$. For incident total <energy> $E$,
$$
s=(p_1+p_2)^2=2m_e^2+2m_eE=16m_e^2,
\qquad \boxed{E_{\rm th}=7m_ec^2=3.577\,\mathrm{MeV}\simeq3.58\,\mathrm{MeV}}.
$$
The corresponding incident <kinetic energy> is $6m_ec^2=3.066\,\mathrm{MeV}$. At threshold the products have no relative <momentum> in the <centre-of-momentum frame>; their common laboratory motion requires more <energy> than merely the pair's rest <energy>.
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