I am confused

(I have been asked for more explanation)

Say if have  

3x+5x-18x  these are all like terms so I can add them    (3+5-18)x = -10x

They are like terms because they are all a number followed by the same letter

5x= 5*x = (2+3)x  that would be true too.

6(x+7)-2(x+7) = 6 lots of (x+7) - 2 lots of (x+7) = (6-2) lots of (x-7) = 4(x-7)

3(2y+7)+5(2y+7)+x(2y+7)

each time exactly the same thing is in the bracket so you have 

3 lots of the bracket +5 lots of the bracket + x lots of the bracket.

this is   (3+5+x) lots of the bracket = (8+x)(2y+7)

You will probably need to think about it for a bit  

Hi Sabi92   

Lets look at what you did 

$$\\4a^2\textcolor[rgb]{0,0,1}{(2a+3)}-6a\textcolor[rgb]{0,0,1}{(2a+3)}+7\textcolor[rgb]{0,0,1}{(2a+3)}\\\\ =(4a^2-6a+7)\textcolor[rgb]{0,0,1}{(2a+3)}$$

Yes that is perfect