What is the solution of log4x − 616 = 4? x = −2 x = 2 x = 3 x = 4

This is how to solve for log expressions.
First, do the arithmetic such that all that can be solved is in the other side, besides log. 
There are different properties of log to help you solve log expressions.

log4x − 616 = 4
log4x = 620
Change how it is written to solve for x.
x^620 = 4
This makes no sense so we try again.

Or, this could be solved differently.
log 4x-6 16 = 4
(4x-6) ^4 = 16
256x - 1296 = 16
x = 5.125

Final  x = 5