Let us write down the equation clearly and in a proper manner. This will help in solving the equation very easily.
8^(x + 3) = 16^(x - 1)
(2^3)^(x + 3) = (2^4)^(x - 1)
(2)^3(x + 3) = (2)^4(x - 1)
Then
3(x + 3) = 4(x - 1)
3x + 9 = 4x - 4
4x - 3x = 9 + 4
x = 13
I hope the procedure is clear enough for you to understand. I also hope that this is the answer that has actually come to your desired help.