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You have 3 equations with 3 variables so that is the number one clue this can work.
You have two different ways to solve this: (1) is substitution and (2) is elimination.
If we were substituting you would take the first equation and set it to one variable-
Ex) x=(-783+12y+2y)/7, and then you'd "plug and chug"
However, you have already stated you can only use substitution so this can be even easier if you think about it.
Let's start with the first two equations:
Now you want to get rid of a variable so let's just get rid of "y" because then I only have to change one of the equations.
These are totally different equations and if we subtract them from each other we are still stuck with three different variables- NOT WHAT WE WANT SO, we multiply the second equation by 4 so that we can ELIMINATED "y":
4(7x-2y-12z=-783) --> 28x-8y-48z=-3132
Now we line them up and subtract:
23x -45z=-3010 (equ. 1)
Now you do the same with the other equation. Do you understand it or do you still need some help?