Please help me on this question

$$\small{\text{Geometric sequence :
$ a_1=a,\ a_2=a*r,\ a_3=a*r^2,\ a_4=a*r^3,\ a_5=a*r^4,\ \dots $
}}\\
\small{\text{
$ a_1=a=3 \qquad a = 3 $
}}\\
\small{\text{
$ a_4=a*r^3 = 3*r^3 =192 \qquad 3*r^3 =192 \qquad r^3 =64 \qquad r =4 $
}}\\
\small{\text{
$ a_3=a*r^2 = 3*4^2 = 3*16 = 48$
}}$$

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