Using ParametricPlot will allow you to plot will generate a curve using $f_x$ (in this case $F\left(\theta\right)$) and $f_y$ (in this case $y\left(\theta\right)$) which are both a function of another parameter ($\theta$).
Here we define the functions given in the original question. Because the user did not specify some of the variables, here I defined the functions such all of the variables may be entered as inputs.
F[a_,l_,k_,n_,\[Lambda]_,\[Theta]_] := ((6 Sqrt[3] k (1 - \[Lambda]* n/Sqrt[l^2 - 2 Sqrt[3] a^2 Cos[\[Theta]]]) Sin[\[Theta]] Sqrt[ l^2 - a^2 (2 + Sqrt[3] Cos[\[Theta]] + Sin[\[Theta]])])/(Cos[\[Theta]] - Sqrt[3] Sin[\[Theta]]))
y[a_, l_, k_, \[Theta]_] := Sqrt[-(2 + Sqrt[3]) a^2 + l^2] - Sqrt[l^2 - a^2 (2 + Sqrt[3] Cos[\[Theta]] + Sin[\[Theta]])]
Next using parametric plot we plot $F$ on the y-axis and $y$ on the x axis for multiple values of $n$, ranging from $n=\text{nMin}$ to $n=\text{nMax}$, with $\text{nP}$ controlling the number of desired $n$ values to plot.
plotFunction[a_, l_, k_, \[Lambda]_, nMin_, nMax_, nP_] :=
ParametricPlot[
Evaluate[
Table[{
y[a, l, k, \[Theta]],
F[a, l, k, n, \[Lambda], \[Theta]]},
{n, nMin, nMax, IntegerPart[(nMax - nMin)/(nP - 1)]}]],
{\[Theta], 0, 2 \[Pi]},
AspectRatio -> 1,
PlotRange -> {Automatic, {-250, 250}},
PlotTheme -> "Scientific",
ImageSize -> If[\[Lambda] == 0, 450, 345],
LabelStyle -> {FontFamily -> "Latex", FontSize -> 25},
FrameLabel -> {"y(\[Theta])",
If[\[Lambda] == 0, "F(\[Theta])", None]},
FrameTicks -> {Automatic, If[\[Lambda] == 0, Automatic, None]},
PlotLegends ->
Placed[LineLegend[
Table["n=" <> ToString[i], {i, nMin, nMax,
IntegerPart[(nMax - nMin)/(nP - 1)]}],
LegendLayout -> "Row"], {.5, .075}],
PlotLabel -> "\[Lambda] = " <> ToString[\[Lambda]]]
For this example I plotted $a=7$, $l=18$, $k=1$, as requested in the original question. I plotted $4$ values of $\lambda$, $\lambda = \{0 ,15\}$, and $n=\{0,3,6\}$.
Row[Table[plotFunction[7, 18, 1, \[Lambda], 0, 6, 3], {\[Lambda], 0, 15, 5}]]
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/dkypF.png)
The user should be able to enter the desired values for $n$, and $\lambda$ into the function as desired.
ParametricPlot[{y, F}, {θ, 0, 2Pi}]
, but you will have to replace your variableλn
with a number first. $\endgroup$TrigFactor[Sqrt[3] Cos[\[Theta]] + Sin[\[Theta]]] == 2 Cos[\[Pi]/6 - \[Theta]]
; next, you can use the second equation to solve fortheta
as a function ofy
and substitute back in the first equation. $\endgroup$