a traingle has a side of 36 and 81 us the pythagoram therom

I assume you want to find the missing side

If these are the legs, the missing side is the hypotenuse given by :

sqrt ( 36^2 + 81^2 ) ≈ 88.64

If the missing side is a leg, we have

sqrt(81^2 - 36^2 ) ≈ 72.56