Question 241679
The first thing you did wrong is eliminate the numerators.
a + b - 2c = 9
3a - 2b + c = -1
2a - b + 3c + 7
---------------------------
You need to eliminate one of the terms. 
1/a+1/b-2/c=9 add this equation to the third equation
3/a-2/b+1/c[=]-1
2/a-1/b+3/c=7
1/a+1/b-2/c=9
-------------------
3/a+1/c=16 #  now subtract equation 2
-3a+2/b-1/c=+1
------------------
2/b=17
17b=2
b=2/17 ans.
1/a+1/(2/17)-2/c=9
1/a+1*17/2-2/c=9
1/a-2/c=9+17/2
1/a-2/c=18/2-17/2
1/a-2/c=1/2 multiply the equation marked with a # by 2 & add.
6/a+2/c=32
------------
7/a=31.5
32.5a=7
a=7/32.5
a=14/65 ans.
2/(14/65)-1/(2/17)+3/c=7
2*65/14-1*17/2+3/c=7
130/14-17/2+3/c=7
3/c=7-130/14+17/2
3/c=(98-130+119)/14
3/c=87/14
87c=3*14
87c=42
c=42/87
c=14/29 ans.
I'll leave the proof up to you.