i want the answer or method for solving this thing!

I'm guessing you mean that log_{3}(243) = p? If so, then we can solve it this way:

By saying that log_{3}(243) = p, what you are in effect saying is that 3^{p} = 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 log_{7}(29) = p; then, the only way to exactly determine p is log(29)/log(7).