Answer:
r = 7
Step-by-step explanation:
The common ratio r of a geometric sequence is
r = [tex]\frac{a_{2} }{a_{1} }[/tex] = [tex]\frac{a_{3} }{a_{2} }[/tex], that is
r = [tex]\frac{21}{3}[/tex] = [tex]\frac{147}{21}[/tex] = 7
