二元一次方程

a/2+b/3=13....(1)
a/3-b/4=3....(2)

(1)*3 + (2)*4 :

3a/2 + b + 4a/3 - b = 39 + 12

3a/2 + 4a/3 = 51

9a + 8a = 51*6

17a = 306

a = 18

代入(1) :

18/2 + b/3 = 13

b/3 = 13 - 9

b = 12