Which formula can be used to describe the sequence? 1.2, 3, 7.5, 18, 7.5,

The given sequence is 1.2, 3, 7.5, 18.75, ...,
This is a geometric sequence with common ratio of 2.5, because
3/1.2 = 2.5
7.5/3 = 2.5
18.75/7.5 = 2.5

The 1st term is   a₁ = 1.2(2.5)⁰
The 2nd term is a₂ = 1.2(2.5)¹
The 3rd term is  a₃ = 1.2(2.5)²
and so on

Therefore the sequence obeys the formula
f(x) = 1.2(2.5)ˣ⁻¹