I think you meant (y+7)+3y(y+7)+(y+7). Anyways, we have to count the number of (y+7) there are. The first term tells us we have 1 group. The second term tell us we have 3y groups. The last term tells us we have 1 group. Therefore we have 1+3y+1 groups of y+7 which gives us (3y+2)(y+7)
