What is 6*5*4*7+z=9831

Z=8711

6*5=30

30*4=160

160*7=1120

9831-1120=8711

6*5*4*7+z=9831

We move all terms to the left:

6*5*4*7+z - (9831) = 0

z-9831+6*5*4*7=0

We add all the numbers together, and all the variables

z-8991=0

We move all terms containing z to the left, all other terms to the right

z=8991