You have not given an equation.
If we make it an equation, then we can solve for x.
a = 5*ln(x +8)
a/5 = ln(x +8) . . . . . . . divide by 5
e^(a/5) = x +8 . . . . . . take the antilog
e^(a/5) -8 = x . . . . . . . subtract 8
If your expression is equal to "a", then x is ...
x = e^(a/5) -8
