subtract 243 base 5 from 243 base 6?

Answer 1

I would change both to base 10, then do the arithmetic.
243₅ = 2(5^2) + 4(5) + 3 = 73
243₆ = 2(6^2) + 4(6) + 3 = 99

now do the subtraction
You don't say what base the answer is to be.