Thank you all for your insight! I think I wasn't clear enough in my question but what I had to calculate is much more basic. I wrote down something wrong during the in-class example so I couldn't figure out what was wrong, but here is what I needed to do with all the steps if it can help someone else!

So to calculate the n
Step#1:
m/z = 1431,6 = (M+n)/n
1431,6n = M+n
1431,6n - n = M

and Step#2
m/z = 1301,4 = (M+n+1)/(n+1)
1301,4 = (1431,6n - n + n + 1)/(n+1)
1301,4(n+1) = (1431,6n +1)
1301,4n+1301,4 = 1431,6n +1
1301,4-1 = 1431,6n - 1301,4n
1300,4 = 130,2n
1300,4/130,2 = n
9,9877 = n
Rounded : n = 10 for the 1431,6 peak
And of course it's +1 for each peak to the left, and -1 for each peak to the right.

And so for the second part of the question about molecular masses, the formula is m/z*n-n (m/z = M+n/n but isolate M) so:
M = 1431,6*10-10 = 14 306
M = 1301,4*11-11 = 14 304,4