<s>[INST] Systems of equations with three variables Consider:
$a + b = 5$
$2a + b + c = 4$
$a - b - c = 5$
I like to use substituiton for solving systems of equations, so I firstly look at equation 1 and solve for $a$
$a + b = 5$
$a = 5 - b$
I substitute this into equation 2
$2(5-b) + b + c = 4$
$10 -2b + b + c = 4$
$-b = 4 - c - 10$
$b = -4 + c + 10$
What next? Am I on the right track? Can I use substitution method for any simultaneous equation?
 [/INST] You have the right concept, and we can see from three equations and three unknowns that these variables are solvable (taking into account equations are linearly independent)
equation 1: $\ a+b=5$
equation 2:$\ 2a+b+c= 4$
equation 3:$\ a-b-c =5$
lets solve for b first:
from equation 1 and 3, we say
$a=5-b$ and $\ c = a-b-5$ 
Now we plug these two 'new' equations into equation 2:
$\ 2(5-b)  + b + ((5-b) - b -5) = 4 $
simplifying for b gives that $ b=2$.
Now taking that b=2, we can sub that back into equation 1 and solve for a and once we have a we can use a and b to solve for c. Which I'm sure you'll be able to solve for it.
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