Question 191760
1)  3a  -4b  =  1
2)  12a  - b = -11

mult  (2)  by  4

2a)  48a  -4b  =  -44

subst  (1)   from  (2a)

45a  =  -45

a  =   (-1)

subst  in  (1)   

3(-1)  -  4b  =  1

-3  -4b  =1

-4 =  4b

b=  (-1)

check

3 (-1)  - 4(-1)  =1
1  =1      ok

12(-1)  -  (-1)  =  -11

-11  =  -11  ok