If you get the units right without cheating, you got the calculation right, too. I often forget formulas or they are sometimes hard to find from a simple Wikipedia lookup, but units are easier to remember. Work out the formula from the units.
Unit of length is m, and unit of area is m^2, so if you forget on which sides of division they are, resulting unit ends up wrong. Repeat until units are correct, and your temperature difference does not contain 1/m, m or m^2 in it; unit of temperature is just K. Not 1/K, for example!