Solve function with a range variable

I have an two variable equation.  I want to use one of the two variables as a range variable, and I need to solve the equation for the second variable.  I want to use the solution later as a function of the range variable.

Very simplified example of equation I want to solve:  R-T

I want to use R as a range variable, but get the solution for T for use later in the work sheet

I have attached an example of the two variable equation.  In this example, I solved for D but R is defined.  I want to use R as a range variable and use the solution D as a function of that range variable.