Imma try my best...
Since 1/8 equals 0.125, I'll just use that.
0.125*x= -4,200
First, we want to isolate x, so we have to divide 0.125*x by 0.125, so it cancels out the 0.125. If we only do it to one side, it becomes unbalanced, so we have to do the same to -4200.
x= -33,600
This should be correct, but let's check.
0.125*x=-4200
0.125*-33,600 =? -4,200
Using calculator to multiply...
-4,200=-4,200
So that should be correct unless there's some special highschool-y math stuff going on...