[tex]f(x)=\log(4x+8x\ln2)=\log(4x(1+2\ln2))=\log4x+\log(1+2\ln2)[/tex]
The function is defined for [tex]x>0[/tex]. The derivative is
[tex]f'(x)=\dfrac4{4x}=\dfrac1x[/tex]
so for any [tex]a>0[/tex], the function's instantaneous rate of change is [tex]\dfrac1a[/tex].