Factor by grouping 8x+8y+rx+ry
i do not understand this.

FACTORISATION IS BASICALLY FINDING LIKE-TERMS.
1. FIND THE HCF OF 8. HERE, THE HCF IS 8. ALSO, CHECK ALL THE LIKE-TERMS. LIKE-TERMS ARE OUTSIDE THE BRACKET AND UNLIKE TERMS ARE INSIDE.
2. ADD THE TERMS IN THE BRACKET AND THE TERMS OUTSIDE THE BRACKET FROM STEP 1.

8x + 8y + rx + ry
1. 8 (x + y) + r (x + y)
2. (x +y) (8+r)