19.3`pi` - 15.8`pi` =`pi`(19.3-15.8)=`pi` `xx`3.5 

                                   = `22/7 xx 7/2`

                                   = 11

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

ax + ay -x -y=a(x+y) - (x+y) 
                    =(x+y) (a-1).

 Notice that when the last two terms are grouped with a minus sign outside. the signs of both terms in the bracket must be changed.

                    ax + ay -x -y=a(x+y) - (x+y) 
 and not     
                   a(x+y) - (x-y) .

It is good plan to remove the brackets mentally to check your working.

`xx^2`y - `xxy^2`+(3y -3`xx`) = xy(x-y) +3(y-x)

This has no obvious factor as the expression inside the brackets are different .
  but if we arrange the expression as 
   
`xx^2`y - `xxy^2`-3`xx`+3y) = `xx`y(x-y) -3(x-y),

An obvious factor is now apparent.
The factors are (x-y)(xy-3).

`xx^2`+9x+20

=`xx^2`+5x +4x +20

=x(x+5)+(x+4)

=(x+5)  (x+4)