What is the diagonal length of a square with 15 feet sides

I am pretty sure you have to do a-squared plus b-squared equals c-squared. That would be 15squared plus 15 squared equals c-squared. 225 plus 225 equals c-squared. 450 equals c-squared or radical 450. So the diangnol would be radical 450 but it should be simplified unless your teachers is OK with it being a radical or squared. Hope I am right/