You are apparently starting with:
f = @(x, a, b, c) Something;
although I have no idea what ‘Something’ may be.
I would do this:
x0 = ...; % Initial Estimate For ‘x’
Xval = fsolve(@(x) f(x,a,b,c) - 2, x0); % Solve For ‘f(x) = 2’
You may be able to use fzero for this as well.
