I am not sure what kind of equation or system of equations you are trying to solve.
Let me state first that the equation you want to solve have to be written using the boolean (fat) equal sign. You get from the Boolean Toolbar or using the appropriate keyboard shortcut (Ctrl-+).
I see three assignments which could qualify as equations. But that way we have some problems. The first and third equations are identical, so we have only two independend equations, but three variables, x, sigma.cy and sigma.sy. So if this is what you want you have to solve for all three variables and maybe you need also a third equation if you expect a unique solution. But guess you have something different in mind.
