If a = m / N + n / m, B = m / N-N / m, find the value of a-square-b-square

If a = m / N + n / m, B = m / N-N / m, find the value of a-square-b-square

a = n/m +m/n
b = n/m -m/n
a²-b²
= (a+b)(a-b)
= (n/m +m/n+n/m -m/n)(n/m +m/n -n/m +m/n)
= 2n/m × 2m/n
= 4