1.3x+2y+z=13
x+y+2z=7
2x+3y-z=12
2.x+2y+3z=11
x-y+7z=10
x+3y+2z=2
3.若方程组4x+3y=1
ax+(a-1)y=3的解,x、y相等,则a的值为_______
A.4B.10C.11D.12
请说出答案以及解题思路