a5 (I think you meant u5. At least, that's what I'm used to writing) = 532.48
r = 8
Sn = a [ r^n - 1 ] / [ r - 1 ] = 0.13 [ 8^n - 1 ] / 7
You can't find out the sum up to n elements here unless you know the actual value of n, or unless you were given some other information that would allow you to derive the value of n. Because this is a diverging geometric series (since r > 1). If it was converging, I would understand how you could be asked to find Sn, because then it would equal S to infinity.
If you were asked to find Sn in terms of r, then that's what I wrote.
Sorry if I'm mistaken, though. good luck.
