Question

Solve and check.
-x - 6 = -4x - 21

Answer 1

Add the 4x to the left side and add the 6 to the right side to get,
3x=-15 ; divide by 3
x=-5
check:
-(-5)-6=-1
-4(-5)-21= -1
-1=-1
Therefore the solution is correct.

Answer 2

-x-6=-4x-21
transpose -x to the other side and 21 to the other side

21-6=-4x+x
15=-3x
divide both side by -3
x=-5