Question 2201
1.   9(x-5)-x(x-5)
  =  (x-5)(9-x)  Answer.

2    ax+ay+2bx+2by
  =  a(x+y)+2b(x+y)
  =  (x+y)(a-2b)  Ans.

3.   ma+mb+na+nb
  =  m(a+b)+n(a+b)
  =  (a+b)(m+n)  Ans.

     Try to take out the common term first.  In the first problem, (x-5) is the common term. Take the common term as one factor and take the coefficients as the other factor. Here, in this case, 9 and -x are the coefficients. Take it as it is in order which comes to (9-x).  That's all.
     In the 2nd & 3rd problems, split out the expression into 2 terms each in such a way that in each two terms you have a common variable and factorise them.
Then again, take out the common term as one factor and the coefficients as another.
      I hope you understood the basic principle of solving such problems. O.K