Question: factorise 2mk - m - p (1 - 2k) by grouping.
2mk - m - p (1 - 2k) --- take out a factor of m from the 1st two terms
m(2k - 1) - p(1 - 2k) --- change the sign of the 1st product
-m(1 - 2k) - p(1 - 2k) --- take out the common factor of (1 - 2k)
(1 - 2k)(-m - p) --- adjust the sign
(2k - 1)(m + p)