This is fairly simple. Divide 2,871,000 by 10 and that's your answer!
Also, since you're dividing by a factor of 10 (or in this case, <em>10</em>) you can just take away as many places as are zeros in the number you're dividing from. This works for <em>10, 100, 1,000, 10,000, etc. </em>In this case, since there is only one zero in 10 you only take away one place marking, which gives you 287,100.
In the step after -6x+10=4x+20, he moved the terms incorrectly. you have to add 6x to both sides so it would be 10=4x+6x+20 which simplifies to 10=10x+20. subtract 20 from both sides to get -10=10x. divide both sides by 10. the answer is x=-1