Solution
= Solution
{c}
Use the <Mandelstam variables> $s=(p_1+p_2)^2$, $t=(p_1-p_3)^2$ and $u=(p_1-p_4)^2$. The external <on shell> relations are $p_1^2=p_2^2=m_e^2$ and $p_3^2=p_4^2=m_t^2$. Expanding the three <Mandelstam variables> and using <four-momentum conservation>, $p_3+p_4=p_1+p_2$, gives
$$
\begin{aligned}
s+t+u
&=3p_1^2+p_2^2+p_3^2+p_4^2+2p_1\cdot(p_2-p_3-p_4)\\
&=p_1^2+p_2^2+p_3^2+p_4^2\\
&=\boxed{2(m_e^2+m_t^2)}.
\end{aligned}
$$