Question 181773
Factor by grouping:
{{{1-n-m+nm}}} Group as shown, and remember to change the sign (+ to -) in the last term:
{{{(1-n) - (m-n*m)}}} Factor the m in the second group.
{{{(1-n)- m(1-n)}}} Now factor the (1-n)
{{{highlight((1-n)(1-m))}}}...and you're done!