Hello, thank you for taking the time to post your question!
When the base isn’t explicitly listed on a logarithm question, it’s safe to assume that the value should be base 10, so we’re trying to find log base 10 of 21. The best way then is to use a calculator in order to get an estimate for what the value should be. When I do that I get log(21) = 1.322219295
So rounded to the nearest tenth place that would be 1.3
I hope that helps get you moving in the right direction! Feel free to reach out if you have any additional questions beyond that :)
