For any arithmetic progression a1,a2,a3,a4,…a_1, a_2, a_3, a_4, \dotsa1,a2,a3,a4,…, what is the value of the expression a1−3a2+3a3−a4a_1 - 3a_2 + 3a_3 - a_4a1−3a2+3a3−a4?
000
ddd
a1a_1a1
−2d-2d−2d