M=M1+M2
−iMe−e−=−i(e2(p∗1+p′∗1)μ(p∗2+p′∗2)μ(p′∗2−p∗2)2−e2(p∗1+p′∗2)μ(p∗2+p′∗1)μ(p′∗1−p∗2)2)
Me−e−=e2(P∗1P∗2+P′∗1P′∗2+P′∗1P∗2+P∗1P′∗2(P′∗2−P∗2)2−P∗1P∗2+P′∗2P′∗1+P′∗2P∗2+P∗1P∗1(P∗1−P∗2)2)
Using the fact that P′∗1P′∗2=P∗1P∗2P′∗1P∗1=P′∗2P∗2P∗1P′∗2=P∗2P′∗1
Me−e−=e2(2P∗1P∗2+2P′∗1P∗2(P′∗22−2P′∗2P∗2+P∗22)−2P∗1P∗2+2P∗1P′∗1(P′∗21−2P′∗1P′∗2+P′∗22))
Me−e−=e2(2P∗1P∗2+2P′∗1P∗2(P∗22−2P∗2P′∗2+P′∗22)−2P∗1P∗2+2P∗1P′∗1(P′∗22−2P′∗2P′∗1+P′∗21))
Me−e−=e2(2P∗1P∗2+2P′∗1P∗2(P∗2−P′∗2)2−2P∗1P∗2+2P∗1P′∗1(P′∗2−P′∗1)2)
Me−e−=e2((P∗21−2P∗1P′∗2+P′∗22)−(P∗21+2P∗1P∗2+P∗22)t−(P∗21−2P∗1P′∗1+P′∗21)−(P∗21+2P∗1P∗2+P∗22)u)
Me−e−=e2(u−st+t−su)