Hi! I'm happy to help!
To solve this, we just have to insert -3 for x. Because the 5 in parenthesis is next to the eight with no other sign, it represents multiplication. Since the eight is next to the x with no other sign, it also means multiplication.
(5)8x
(5)·8·x
We can remove the parenthesis.
5·8·x
5·8·-3
40·-3
-120
So, when x=-3, (5)8x evaluates to -120.
I hope this was helpful, keep learning! :D