Remember - if |r| < 1, then the series converges and has a sum (goes down). But, if |r| > 1, then the series diverges and does not have a sum (goes up).
With this series - we can find r by dividing - a₂/a₁, or 28/7 = 4.
Because r = 4 and 4 > 1, the series will diverge and will have no sum because diverging sequences have no sum.