Hello, thank you for taking the time to post your question!
The default assumption that you want to use here is that this is a base-10 logarithm. That means you can rewrite the equation as
10^2 = 3x + 5
Solving that for x then I get
100 – 5 = 3x
95 = 3x
x = 95/3
I hope that helps get you moving in the right direction! Feel free to reach out if you have any additional questions beyond that :)