First we move the "2" to the right of the equal sign, like this:
101 - 10 = 21.
Second, we read these numbers in base 3:
101 base 3 = 10 base 10
10 base 3 = 3 base 10
21 base 3 = 7 base 10
So, we have the expression (base 10): 10 - 3 = 7, that is true.