3 equation - simultanious equations?

Equation 2-1 -2 =3a +b, b= -2 -3a
Equation 3-1 -4 =15a +3b

-4 =15a +3(-2 -3a)
-4 =15a -6 -9a
2 =6a
a =1/3

Sub into Eq 2-1 -2 =3(1/3) +b, b=-3

4 =1/3 -3 +c
c = 20/3

a=1/3, b=-3, c=20/3

thefirst please tell me first what that ok=0 for? Subtract every physique of them from the different 2 to get rid of c and u get 2 eqn in a and b. resolve them (a minimum of) and the use those values of a and b in every physique eqn to locate c.

- 4 = - a - b - c
2 = 4a + 2b + c-----ADD

- 2 = 3a + b

- 2 = - 4a - 2b - c
0 = 16a + 4b + c--------ADD

- 2 = 12a + 2b

-2 = 3a + b
-2 = 12a + 2b

4 = - 6a - 2b
-2 = 12a + 2b-------ADD

2 = 6a
a = 1/3

4 = - 2 - 2b
2b = - 6
b = - 3

4 = 1/3 - 3 + c
c = 7 - 1/3
c = 20/3

a = 1/3 , b = - 3 , c = 20/3

4 = a + b + c
2 = 4a + 2b + c
0 = 16a + 4b + c

4 = a + b + c
a = 4 - b - c

2 = 4a + 2b + c
2 = 4(4 - b - c) + 2b + c
16 - 4b - 4c + 2b + c = 2
-4b + 2b - 4c + c = 2 - 16
-2b - 3c = -14

0 = 16a + 4b + c
16a = 0 - 4b - c
16(4 - b - c) = 0 - 4b - c
64 - 16b - 16c = 0 - 4b - c
-16b + 4b - 16c + c = 0 - 64
-12b - 15c = -64

-2b - 3c = -14
6(-2b - 3c) = 6(-14)
-12b - 18c = -84

...-12b - 15c = -64
-- -12b - 18c = -84
-----------------------------
3c = 20

3c = 20
c = 20/3

-12b - 18c = -84
-12b - 18(20/3) = -84
-12b - 120 = -84
-12b = -84 + 120
-12b = 36
b = 36/-12
b = -3

4 = a + b + c
4 = a + (-3) + 20/3
a = 4 - 20/3 + 3
a = 12/3 - 20/3 + 9/3
a = 1/3

∴ a = 1/3 , b = -3 , c = 20/3