l[y_] := D[y, {t, 2}] + 3  D[y, t] + 2  y;

\(\(
\(Clear[y1, y2]; \)\)\)

y1[t_] := E^(-t);  y2[t_] := E^(-2 t);

l[y1[t]]

l[y2[t]]

\(\(
\(w[f1_, f2_] := Det[{{f1, f2}, \ {D[f1, t], D[f2, t]}}]; \)\)\)

w[y1[t], y2[t]]

w[y1[x], y2[x]]

\(\(
TrigExpand[Sin[3  t]]\)\)

% /. {Cos[t]^2 -> 1 - Sin[t]^2}

% // ExpandAll

% // Simplify

\(\(
x + 2  x^2 + 3  x^3 /. {x -> y}\)\)

% /. {y -> y^2}

% /. {y^2 -> y^4}

\(\(
\(g[x_] := E^\((I\ x)\); \)
\)\)

ListPlot[{Re[g[x]], 
          Im[g[x]]} /. {{x -> 1}, {x -> 2}, {x -> 3}, {x -> 4}}]