If the start temperature is t°C, and the temperature after 1 hour is t^2°C, we already know the equation to use to find the after temperature from the start temperature, which is t•t or t^2, so to find what the start temp is from the temp after 1 hour we reverse it to find the square root. So we find the square root of 81, which is 9, we have our answer.
t=9 so thats our answer!