Question 334977
No, 6x^3 and 2x^2 are not like terms. So you cannot add them. So it doesn't make sense to stack the two vertically. 



(6x^3 -5x -4) - (2x^2 - 8x + 3) ... Start with the given expression


6x^3 -5x -4    -2x^2 + 8x - 3 ... Distribute.


6x^3 +0x^2-5x -4    -2x^2 + 8x - 3 ... Add a 0x^2 term in between the first two terms.



Now stack the first four terms over the last three.


<pre>
6x^3+0x^2 -5x -4
    -2x^2 +8x -3
</pre>


Now simply add the terms that line up. I'll let you do that.