Answer:
lg21/(3lg2)
Step-by-step explanation:
log8(21)
Common log: log10 or lg
logb(a)
Change of base law:
logc(a)/logc(b)
log8(21)
lg21/lg8
8 = 2³
log8 = lg2³ = 3lg2
lg21/(3lg2)
By "common logarithm" you probably mean the natural logarithm. And you'll need the formula to change base:
[tex]\log_a(b)=\dfrac{\ln(b)}{\ln(a)}[/tex]
So, you have
[tex]\log_8(21)=\dfrac{\ln(21)}{\ln(8)}[/tex]