How do I solve "-2n-8n+42=0" using the quadratic formula?
as written, you can't. If you mean
-2n^2-8n+42=0
then get rid of the factor of -2, and you have
n^2+4n-21 = 0
n = (-4±√(4^2+84))/2
...
Just use the formula and plug in your numbers. Sometimes it helps to factor something out, so you don't have to work with large values.
And factoring out -2 instead of 2 is just my personal preference, so I can get a positive coefficient for n^2. It does not affect the final answer in any way.