I just started using Mathematica in February, learning & enjoying (when I can). Recently i've just began exploring the manipulate function, while the manipulate part itself works fine, I have to manually change the f[x_] for each equation I want to explore. In order to export to CDF or just simplify the looks I wanted to use an InputField or similar method to express an equation but I can't seem to figure out how to go from InputField to an equation the manipulation expression will work with. The working code I have is:

f[x_] := {{XXXX}}

g[a_] := Evaluate[D[f[a], a]]

Manipulate[Plot[{f[x], g[x], g[a] *(x-a) + f[a]}, {x, c,-1*c},

PlotRange -> {b, -1*b},

PlotLedgends -> {f[x], g[x], "f'(x) m value" -> g[a]}], {{a, -5}, {-10,10},

{{b,5}, 0.1,10}, {{c, 5}, .1,10}]

//Replace {{XXXX}} the equation

All it does is plot a function, its derivative and the tangent line that tracks the original function. I was trying to define the function via InputField and despite my best efforts I can't crack it.