Theory:

Factorisation by taking out the common binomial factor from each term of the algebraic expression.
Consider n2+1(mn)+(m2+1)(mn).
 
Take binomial factor from each term commonly outside.
 
n2+1(mn)¯+(m2+1)(mn)¯
 
=(mn)(n2+1+m2+1)
 
=(mn)n2+m2+2