You could use a Mod function to sample the area you have the function defined over at other parts of the x$x$-axis if thatsthat's what you're aiming for.
f[t_] := Piecewise[{{3 t, 0 <= t <= Piπ/2}, {3 t + 6, Piπ/2 <= t <= Piπ}}] Plot[f[Mod[t, \[Pi]]]π]], {t, -\[Pi]π, 2 \[Pi]π}] 
You could use a Mod function to sample the area you have the function defined over at other parts of the x-axis if thats what you're aiming for.
f[t_] := Piecewise[{{3 t, 0 <= t <= Pi/2}, {3 t + 6, Pi/2 <= t <= Pi}}] Plot[f[Mod[t, \[Pi]]], {t, -\[Pi], 2 \[Pi]}]
You could use a Mod function to sample the area you have the function defined over at other parts of the $x$-axis if that's what you're aiming for.
f[t_] := Piecewise[{{3 t, 0 <= t <= π/2}, {3 t + 6, π/2 <= t <= π}}] Plot[f[Mod[t, π]], {t, -π, 2 π}]
