i want the answer or method for solving this thing!
I'm guessing you mean that log3(243) = p? If so, then we can solve it this way:
By saying that log3(243) = p, what you are in effect saying is that 3p = 243. Replace p with a succession of integers and you will find that p = 5.
But what if p isn't an integer? Say we have log7(29) = p; then, the only way to exactly determine p is log(29)/log(7).