Because it is an isosceles triangle, the two longer sides are the same length. Therefore multiply 6.3 times 2. To find the length of the base subtract 12.6 from 15.7 to get 3.1 cm.
There is no sum for this geometric series. it diverges rather than converges due to the absolute value of the common ratio (r), which is -3, being 3. for a geometric series to have a sum (to converge), the absolute value of r must be less than 1. (you find r by dividing a2/a1, a3/a2, etc.) hope this helps