To answer this question, use the following rule of logarithms:
`log_a x^b = blog_a x`
This means that logarithm of any base of a number taken to the power b equals b multiplied by the logarithm of the same base of that number.
In this case, the number is 3, and the logarithm of the base a of 3 is given: `log_a 3 = 1.618`
The number 81 is a power of 3: ` `
`3^4 = 81`
So, the logarithm of base a of 81 can be rewritten, using the above rule, as
`log_a 81 = log_a 3^4 = 4log_a 3`
and calculated as 4(1.618) = 6.472.
Therefore, the value of