Q:

Using solution of an equation

I want to solve something like f(x)=0 and then have the real solution be set as the value of another variable. I can't figure out how to do it. I tried something like this as an example, but it didn't work.
a = Solve[x + 1 == 0, x, Reals]

added: Why won't my InputField dynamically update?
InputField[ Dynamic[G = x /. Solve[ 2 == x*Cos[\[Theta]] -(P*x^2)/(6*C*D) (3 F + x) Sin[\[Theta]], x, Reals]]]

second add: here's the rest of my code.
    Manipulate[
 DynamicModule[{P = 0, C = 1, D = 1, F = 0, θ = 0}, 
  Deploy[Style[
     Panel[Grid[Transpose[{{"P", "C", "D", "F", "θ", "G"},
     {InputField[Dynamic[P]], InputField[Dynamic[C]], 
      InputField[Dynamic[D]], InputField[Dynamic[F]], 
      InputField[Dynamic[θ]], 
      InputField[
       Dynamic[
        G = x /. 
          Solve[2 == 
            x*Cos[θ] - (P*x^2)/(
              6*C*D) (3 F + x) Sin[θ], x, Reals][[1]]]]}}],
   Alignment -> Right], ImageMargins -> 10, DefaultOptions ->
   {InputField -> {ContinuousAction -> True, 
      FieldSize -> {{5, 30}, {1, Infinity}}}}]]] Dynamic[Show[
 {Graphics[{Opacity[0.5], Red,
    Rectangle[{1, 0}, {2, 1}]},
   PlotRange -> {{-1, 2}, {-3, 3}}, Axes -> True,
   AxesOrigin -> {0, 0}],
  ParametricPlot[{x*Cos[θ] - (P*x^2)/(
      6*C*D) (3 F + x) Sin[θ], 
    x*Sin[θ] + (P*x^2)/(6*C*D) (3 F + x) Cos[θ] + 
     G*Sin[θ] + (P*G^2)/(6*C*D) (3 F + G) Cos[θ] - 
     2}, {x, 0, B}, Axes -> True]}]]]]

A:

You want to run
a = x /. Solve[x + 1 == 0, x, Reals][[1]]

The [[1]] selects the first solution, and the x /. applies the solution to the expression x.

