What is the base of log (x+7)

It depends upon the 'base" of your log. Let us say that is 10; we can write:

log _10 (x + 7) = 1

using the definition of log we can write:

x + 7 = 10 ^1

and:

x = 10 - 7 = 3

if you have a different base use the one you got instead of 10.

10

Step-by-step explanation:

Generally we use the base 10 to any logarithmic function, which is known as common Base

so the base of log (x+7) is 10