3 - 4x = 6X + 13
3-4x (+4x) = 6X+13 (+4x)
3 = 10x + 13
3 (-13) = 10x + 13 (-13)
-10 = 10x
-10/10 = x
-1=x
But suppose x were -5.
3 - (4 * -5) would be -17.
(6 * -5) + 13 would also be -17.
So why is your answer right and mine....er, whatever.
You worked it as I would have..we must have had the same beginning algebra book :)