Question

How do you find `a_4` if `a_1=-7` and `r=-3/4`?

Answer 1

From a1 to a4 is three steps, so you multiply by -3/4 three times

This is what we call an arithmetic series. Every next item is calculated by multiplying the one before by the multiplicant r.
so a1=-7 a2=-7 x-3/4 and so on
Or in short (three steps!) a1 x r^3 = (-7)*(-3/4)^3