9x+3y-4z=35

Question

x+y-z=4
2x-5y-4z+48=0

Answer 1

I assume you are trying to solve this system.

9x+3y-4z=35 **
x+y-z=4 ***
2x-5y-4z = -48 ****

subtract **** from **
7x + 8y = 83 ##
4 times *** ----> 4x + 4y - 4z = 16
subtract **** from the above
2x + 9y = 64 ###

7 times ### ---> 14x + 63y = 448
2 times ## ----> 14x + 16y = 166
subtract those last two
47y = 282
y = 6

sub that into ## to get x

Now you have x and y,
sub that into *** to get z