1. The problem statement, all variables and given/known data
I need help solving for d, in the really long equation below. However, whenever I insert the following command (see below), I get 'is not a valid equation or expression', but I can't figure out why.


2. Relevant equations
((cot(104.789d))^2) + (((cot(104.789d))^2)*(1 + (-1.339*(10^-3)))*((sin(209.58d))^2))=-0.3939


3. The attempt at a solution
solve('((cot(104.789d))^2) + (((cot(104.789d))^2)*(1 + (-1.339*(10^-3)))*((sin(209.58d))^2))=-0.3939','d')