In the equation as written, if A and B are unitless constants, then T has to be dimensionless. Otherwise you would be trying to add a temperature to the square of a temperature, an operation that makes no sense and is not allowed by Mathcad.
Assuming that that unit expression tacked on at the end represent the units that you want for the result, then you need for C to have the units of the result, W/(m·K), B must have units of that over K, W/(m·K²), and A must have units of W/(m·K³). Or at least units representing those dimensions. You can input the values using any units you wish, as long as the result represents the correct physical quantity.
That is the basis for Mathcad's unit support -- it stores representations of physical quantities, not specific units. Thus it makes absolutely no difference if you define a variable as 1000·m or 1·km -- they both represent the same length, and are stored by Mathcad exactly the same.
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� � � � Tom Gutman