6 + (-3) would be the same as just saying 6-3 which would be equal to positive 3
Answer:
The formula is:

Step-by-step explanation:
The geometric sequences are those in which the division between the terms
and
of the sequence are equal to a constant common reason called "r"
The geometrics secencias have the following form:

Where
is the first term of the sequence
In this sequence we have the following terms
1.2, 3, 7.5, 18.75
Then notice that:

Then:
and 
Finally the formula is:

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