We are given the first term and the common ratio, this means they belong to a geometric series.
For the given series:

Each term of the geometric series is obtained by multiplying the previous term by common ratio.
So the next terms will be:
-4.5, -6.75, -10.125, -15.1875, -22.78125
The general formula for the G.P would be:

On plotting the series, the result will be like this:
Equation: 21+n=43.23
Process to find n
21+n=43.23
Subtract 21 from both sides
n=22.23
3y=2(2+2y); 3y=4+4y; -y=4; y=-4;