Put in 1 or -1 coefficients, and +0y for the missing term in
the 2nd equation.  Put 0x for the missing term in the third
equation:



Form the denominator determinant by using only the coefficients
to the left of the = sign.  You don't use the red numbers in D,
but you will in the others:

 = 21

[Do you know how to evaluate a 3x3 determinant?
If not, post again asking how to.  I will assume
you already know how.  That's where I got the 21.]

x is the FIRST unknown and so  is just like 
except that the FIRST column is replaced by the three red numbers,
which I made up: 

 = -42

y is the SECOND unknown and so  is just like 
except that the SECOND column is replaced by the three red numbers: 


 = 0

z is the THIRD unknown and so  is just like 
except that the THIRD column is replaced by the three red numbers: 

 = 105

So:





Then the solutions are given by:

== = 
== = 
= = 

Edwin