Well I hate to break the news but 243 is not a perfect square. I'll work you through it, 243 is not a perfect square because it is not an even number. an even number must end in (0,2,4,6,8)
Step one. Find the square root. the square root of 243 is 15.588.
Step two Is it a perfect square. No 243 just cant be a perfect square.