I don't get this. Can someone explain it better?

Pick some number . Then multiply this by another number . Keep doing this and you generate a geometric sequence .

The 11th and 12th terms in this sequence are then and (because you get the 11th term after you multiply by 10 times, and the 12th term after 11 times).

The first term is, of course, . We're given and . The sum of the 11th and 12th terms are then computed as shown in your picture.