It doesn't matter, just as long as you explain which number is which.
so the investigator found the skid marks were 75 feet long hmmm what speed will that be?
nope, the analysis shows that Charlie was going faster than 35 m/h.
now, assuming Charlie was indeed going at 35 m/h, then his skid marks would have been
6n^2-5n^2= n^2
n^2+7n^2= 8n^2
Final answer: 8n^2
Remember that all of these terms are like terms so you can just add/subtract coefficients.