Maths problem: how to do 11, thanks?

m - n = 5
2n^2 - m^2 = 71
∴ 2n^2 - (n+5)^2  = 71
   n^2 - 1on - 96 = o
  (n-5)^2 = 121 = 11^2
∴ n = 16
   m = 21