f[x_] := x^2 + c;
y = Solve[f'[f[x]] == 1, x, Reals]
output:
{{x -> ConditionalExpression[Root[-1 + 4 c #1 + 4 #1^3 &, 1],
c > -(3/4) || c < -(3/4)]}, {x ->
ConditionalExpression[Root[-1 + 4 c #1 + 4 #1^3 &, 2],
c < -(3/4)]}, {x ->
ConditionalExpression[Root[-1 + 4 c...