3x+2y=5+2a (1)
2x-3y=16-7a (2)
(2)×14-(1)×5
28x-42y-15x-10y=224-98a-25-10a
13x-52y=199-108a
x-4y=10-3a
两边乘13
13x-52y=130-39a
所以199-108a=130-39a
69a=69
a=1