Please fix your notation. You're all over the place on that on.
If you mean this: , you should have written 5^x = 4^(x+3). Parentheses make a difference.
This is okay, excepting the notation.
log [5^x] = log [4^(x+3)]
Again with notation confusion:
x log 5 = (x+3) log 4
Why are you stuck? Keep going!
x log 5 = x log(4) + 3 log 4
x log(5) - x log(4) = 3 log(4)
x [log(5) - log(4)] = 3 log(4)
Can you finish.
log(4) is a number. It may not be the prettiest number ever, but it's not the ugliest, either. Use it like any other number.