Question 121457
3xsquared+3xy-6ysquared=60...Factorise 
3x(x+2y)-3y(x+2y)=60
(3x-3y)(x+2y)=60....1

 3x-3y can be written as 3(x-y)...2

Substitute x-y=6 in 2 above 
Now 3x-3y=18

go back to 1 above and plug in 18
 
18(x+2y)=60....Divide both sides by 18
x+2y=60/18........3

9x+18y can be written as 9(x+2y)

 9(x+2y)=9times60/18=30