Factorise m2+mn−25m−25n completely.

Sure, let's factorise the given expression step by step.

The given expression is m^2 + mn - 25m - 25n.

Step 1: Group the terms to make it easier to factorise.

= (m^2 + mn) - (25m + 25n)

Step 2: Factor out the common factors in each group.

= m(m + n) - 25(m + n)

Step 3: Notice that (m + n) is a common factor in both terms. Factor out (m + n).

= (m - 25)(m + n)

So, m^2 + mn - 25m - 25n is completely factorised as (m - 25)(m + n).